spi: amd: Use iopoll for busy waiting

Instead of implementing a custom IO busy wait function, just use
readl_poll_timeout().

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Link: https://lore.kernel.org/r/20220211143155.75513-2-andrealmeid@collabora.com


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