scpi-pps: Change some floats to doubles.

scpi-pps: Change some floats to doubles.

This makes the code more consistent with the rest of the code-base
and also allows std_gvar_min_max_step_array() to work here.

Without this change:

  src/hardware/scpi-pps/api.c: In function ‘config_list’:
  src/hardware/scpi-pps/api.c:570:40: warning: passing argument 1 of ‘std_gvar_min_max_step_array’ from incompatible pointer type [-Wincompatible-pointer-types]
      *data = std_gvar_min_max_step_array(ch_spec->voltage);
                                          ^~~~~~~
  In file included from src/scpi.h:30:0,
                   from src/hardware/scpi-pps/api.c:23:
  src/libsigrok-internal.h:964:19: note: expected ‘const double *’ but argument is of type ‘const float *’
   SR_PRIV GVariant *std_gvar_min_max_step_array(const double a[3]);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/hardware/scpi-pps/api.c:573:40: warning: passing argument 1 of ‘std_gvar_min_max_step_array’ from incompatible pointer type [-Wincompatible-pointer-types]
      *data = std_gvar_min_max_step_array(ch_spec->frequency);
                                          ^~~~~~~
  In file included from src/scpi.h:30:0,
                   from src/hardware/scpi-pps/api.c:23:
  src/libsigrok-internal.h:964:19: note: expected ‘const double *’ but argument is of type ‘const float *’
   SR_PRIV GVariant *std_gvar_min_max_step_array(const double a[3]);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/hardware/scpi-pps/api.c:576:40: warning: passing argument 1 of ‘std_gvar_min_max_step_array’ from incompatible pointer type [-Wincompatible-pointer-types]
      *data = std_gvar_min_max_step_array(ch_spec->current);
                                          ^~~~~~~
  In file included from src/scpi.h:30:0,
                   from src/hardware/scpi-pps/api.c:23:
  src/libsigrok-internal.h:964:19: note: expected ‘const double *’ but argument is of type ‘const float *’
   SR_PRIV GVariant *std_gvar_min_max_step_array(const double a[3]);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~