drivers/wifi/nrfwifi: Add buffer for discard bytes

Some spi drivers do not allow the send buffer
and receive buffer to be empty at the same time,
if this happens it will cause the spi to be unable
to communicate with the nrf7002, so add the receive
buffer for the discard byte in the spim_xfer_rx.

Fix #80686

Signed-off-by: Hongquan Li <hongquan.prog@gmail.com>