spi: zynqmp-gqspi: Clean up fillgenfifo

This function does a lot more work (assigning things multiple times,
masking unnecessarily, comparing to zero, using superfluous parentheses)
than it needs to. This makes it difficult to understand and modify.
Clean it up. No functional change intended.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://patch.msgid.link/20250116224130.2684544-5-sean.anderson@linux.dev


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