BTS: refactor f_get_si3(), so it can be used to get SI4

This is needed for the follow up change(s) verifying the GPRS
indicator in the Rest Octets of RR System Information Type 4.

Change-Id: I540b43bbe886f8ca3e9a7eb49a4d30d391d45f49
Signed-off-by: Vadim Yanitskiy <axilirator@gmail.com>
Related: OS#3075
1 file changed