drivers: modem: gsm: allocate in modem_cmd_handler with K_NO_WAIT

All net_bufs allocated to modem_cmd_handler's data->rx_buf are consumed
synchronously in gsm_ppp.c in the same thread. This means that
allocating them with timeout makes no sense, because timeout
will *always* be hit when there are no more buffers in net_buf_pool.

Get rid of the unnecessary timeout, as it doesn't help and just slows
down processing of incoming data, increasing possibility of data
overrun.

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