scpi-dmm: minor style adjustment (gdm906x, analog init, comments, diag)

Address minor style issues: Need not assign NULL after g_malloc0(), need
not check for NULL before g_free(). Rephrase diagnostics messages which
are user visible by default, remove internal development details. Reword
a few comments, and adjust their grammar for consistency across the code
base. The sr_analog_init() routine executed immediately before getting
measurements, need not (re-)assign endianess or floating point details,
except those which do change after initialization (double vs float).
Rephrase model dependent checks for easier adjustment during maintenance.
Unobfuscate string comparisons.