spi: spi-mt65xx: Rename a variable in interrupt handler

spi: spi-mt65xx: Rename a variable in interrupt handler

All the spi_transfer variables in this file use the name "xfer" except
the one in mtk_spi_interrupt(). Align the naming for consistency and
easier searching.

While at it, reformat one memcpy() usage since the coding style allows
100 column lines today.

This commit has no functional change.

Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://msgid.link/r/20240321070942.1587146-3-fshao@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>