spi: sh-msiof: Fix maximum DMA transfer size

spi: sh-msiof: Fix maximum DMA transfer size

The maximum amount of data to transfer in a single DMA request is
calculated from the FIFO sizes (which is technically not 100% correct,
but a simplification, as it is limited by the maximum word count values
in the Transmit and Control Data Registers).  However, in case there is
both data to transmit and to receive, the transmit limit is overwritten
by the receive limit.

Fix this by using the minimum applicable FIFO size instead.  Move the
calculation outside the loop, so it is not repeated for each individual
DMA transfer.

As currently tx_fifo_size is always equal to rx_fifo_size, this bug had
no real impact.

Fixes: fe78d0b7691c0274 ("spi: sh-msiof: Fix FIFO size to 64 word from 256 word")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/d9961767a97758b2614f2ee8afe1bd56dc900a60.1747401908.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>