tbf: Replace static casts by calls to as_ul_tbf/as_dl_tbf

Currently casts from gprs_rlcmac_tbf to gprs_rlcmac_ul_tbf and
gprs_rlcmac_dl_tbf are done by using static_cast. This doesn't provide
protection against converting a gprs_rlcmac_ul_tbf pointer to a
gprs_rlcmac_dl_tbf pointer and vice versa.

This commit provides two functions as_ul_tbf and as_dl_tbf, that
behave similar like dynamic_cast but use the direction field instead
of RTTI.

Sponsored-by: On-Waves ehf
5 files changed