addons/chan_mobile: Increase buffer size of UCS2 encoded SMS messages

When UCS2 character encoding is used, one symbol in national language can be
expanded to 4 bytes. The current buffer used for receiving message in
do_monitor_phone is 256 bytes, which is not large enough for incoming messages.

For example:
* AT+CMGR phone response prefix
  '+CMGR: "REC UNREAD","+7**********",,"14/10/29,13:31:39+12"\r\n' - 60 bytes
* SMS body with UCS2 encoding (max) - 280 bytes
* AT+CMGR phone response suffix '\r\n\r\nOK\r\n' - 8 bytes
* Terminating null character - 1 byte

This results in a needed buffer size of 349 bytes. Hence, this patch opts for a
350 byte buffer.

ASTERISK-24468 #close
Reported by: Dmitriy Bubnov
patches:
  chan_mobile-1_8.diff uploaded by Dmitriy Bubnov (License 6651)
  chan_mobile-trunk.diff uploaded by Dmitry Bubnov (License 6651)
........

Merged revisions 427607 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 427610 from http://svn.asterisk.org/svn/asterisk/branches/12
........

Merged revisions 427611 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@427612 65c4cc65-6c06-0410-ace0-fbb531ad65f3