drivers: modem: ublox-sara-r4: Convert driver to new DT device macros

Convert ublox-sara-r4 driver:

    NET_DEVICE_OFFLOAD_INIT -> NET_DEVICE_DT_INST_OFFLOAD_DEFINE

DT label is already required, so use it and drop
CONFIG_MODEM_UBLOX_SARA_R4_NAME option.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>