drivers: modem: sara-r4: update functions to return POSIX values

The original Ublox SARA-R4 driver was written for the net_context
APIs.  As a result many of the return values are not POSIX standard.

Let's go through all of the socket offload functions and make sure
we return standard values (0 or -1) and set errno where appropriate.

Signed-off-by: Michael Scott <mike@foundries.io>