wifi: nrf_wifi: net_if: allocate memory before mutex

Allocate the memory in `nrf_wifi_if_send` *before* taking the nrf70
global mutex. This prevents the function from deadlocking the
application if attempting to send under memory-pressure, since many of
the memory release paths also happen under the global mutex.

Signed-off-by: Jordan Yates <jordan@embeint.com>