Update modem.py

some modem, report error
-------------
ATI

SIMCOM_Ltd
SIMCOM_SIM300C
Revision:1008B15SIM300C32_SPANSION

OK
AT+CSMP=49,167

ERROR
AT+CSMP=49,167,0,0

OK
Merge pull request #1 from chakphanu/patch-1

Update modem.py