rilmodem: default gprs max_cids to 1 (LP: 1254746)

Some rild implementations ( eg. maguro ) don't always return
a value for max_cids in a reply to a DATA_REGISTRATION request.
Ensure that the sensible default of 1 is used.
Merge pull request #32 from tonyespy/gprs-fix

rilmodem: default gprs max_cids to 1