gsm_04_11: use gsm48_decode_bcd_number2(), evaluate rc

Avoid deprecation warning: use gsm48_decode_bcd_number2() instead of
gsm48_decode_bcd_number().

Validate the return value and add error handling.

Change-Id: Ibef71c46d72d2d43123e68f73e5ed554a69243d8
1 file changed