spi: amd: Refactor amd_spi_busy_wait

Use amd_spi_readreg32 to read 32 bits registers

Signed-off-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210910111529.12539-2-tanureal@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>