spi: sophgo: Fix incorrect use of bus width value macros

spi: sophgo: Fix incorrect use of bus width value macros

The previous code initialized the 'reg' value with specific bus-width
values (BUS_WIDTH_2_BIT and BUS_WIDTH_4_BIT), which introduces ambiguity.
Replace them with BUS_WIDTH_MASK to express the intention clearly.

Fixes: de16c322eefb ("spi: sophgo: add SG2044 SPI NOR controller driver")
Signed-off-by: Longbin Li <looong.bin@gmail.com>
Link: https://patch.msgid.link/20251117090559.78288-1-looong.bin@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>