modem: ublox-sara-r4: use NET_SOCKETS_OFFLOAD_PRIORITY

Use configurable NET_SOCKETS_OFFLOAD_PRIORITY instead of hardcoded value
in the driver itself. This allows to select relative priority of
offloaded TLS versus native TLS when used together with
NET_SOCKETS_TLS_PRIORITY.

Drop the build assert, as always prioritizing offloaded TLS over native
TLS should be application developer choice.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>