spi: dw: fix first spi transfer with dma always fallback to PIO

spi: dw: fix first spi transfer with dma always fallback to PIO

Even with proper dma engine support, the first spi transfer always
fallback to PIO, the reason is the dws->n_bytes is 0 after
initialization, so the dw_spi_can_dma() calling from __spi_map_msg()
return false, thus both tx_sg_mapped and rx_sg_mapped are false, so
for the first spi transfer, the spi_xfer_is_dma_mapped() reports false
thus fallback to PIO.

Although this brings no harm, we can simply fix this issue by
calcuating the "n_bytes" from xfer->bits_per_word.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://patch.msgid.link/20260615044039.9750-2-jszhang@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>