Clarify sp_list_ports() code.

Clarify sp_list_ports() code.

This also fixes the following scan-build warning:

  serialport.c:335:15: warning: Result of 'malloc' is converted to a
  pointer of type 'struct sp_port *', which is incompatible with sizeof
  operand type 'struct sp_port **'
          if (!(list = malloc(sizeof(struct sp_port **))))
                       ^~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~