tbf: Move the current CS field to GprsMs

Currently the current CS value is stored in the cs field of
gprs_rlcmac_tbf and initialised when it is used the first time.

This commit adds separate fields for UL and DL CS values to the
GprsMs class and provides corresponding getter methods for GprsMs and
gprs_rlcmac_tbf.

Ticket: #1739
Sponsored-by: On-Waves ehf
8 files changed