Quell warning from MinGW on setting CM108 pttbitnum

MinGW was issuing the following warning:

  CC     rig.lo
rig.c: In function 'rig_init':
rig.c:330:36: warning: assignment makes integer from pointer without a cast [enabled by default]
  rs->rigport.parm.cm108.ptt_bitnum = DEFAULT_CM108_PTT_BITNUM;
                                    ^

The constant DEFAULT_CM108_PTT_BITNUM was being defined as a string on
non-linux platforms.  Now defined to an integer on these platforms.  The
actual value may need to be corrected at some point in the future when
CM108 support is completed for those platforms.