wlcore: SPI - fix spi transfer_list

In corner case for wl12xx_spi_raw_write() when
	len == SPI_AGGR_BUFFER_SIZE
we don't setup correctly spi transfer_list.
Next we will have garbage and strange errors
reported by SPI framework (eg. wrong speed_hz,
failed to transfer one message from queue)
when iterate transfer_list.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>