drivers: modem: sara-r4: support for 2G on the SARA R412M modem

The SARA R412M requires manual activation of the PDP context. It also
reports RSSI instead of RSRP when on 2G. An off-by-one in the RSRP
calculation was also fixed in this commit.

Signed-off-by: Göran Weinholt <goran.weinholt@endian.se>