Find a real fix for EGPRS_AckNack_w_len_t decoding
Refers to commit: 'edge: Workaround to fix decoding of EGPRS_AckNack_w_len_t'
The presence of the LENGTH field adds an additional offset which breaks the related M_SERIALIZE in gsm_rlcmac.cpp. In that case, the Desc in EGPRS_AckNack_t is being cast to EGPRS_AckNack_w_len_t and then ((EGPRS_AckNack_w_len_t *)Desc)->Desc is filled with the
parsed data, which is a platform dependant number of bytes apart the real Desc struct.
That commit removes the LENGTH field from EGPRS_AckNack_w_len_t so that the Desc field is the first field.
Note that this is not a real fix. The rlcmac wireshark dissector
still has the same declaration but doesn't seem to suffer from this
problem. So there is probably some other bug.