rlc: Add missing break after GPRS_RLCMAC_CONTROL_BLOCK_OPT

Do not claim that the payload is not known. Add the missing break.

Do not print:
<0002> gprs_rlcmac.cpp:1174 GPRS_RLCMAC_CONTROL_BLOCK_OPT block payload is not supported.
<0002> gprs_rlcmac.cpp:1176 Unknown RLCMAC block payload.
1 file changed