spi: SPISG: Fix less than zero comparison on a u32 variable

The check for ns < 0 is always false because variable ns is a u32 which
is not a signed type. Fix this by making ns a s32 type.

Fixes: cef9991e ("spi: Add Amlogic SPISG driver")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://patch.msgid.link/20250725171701.839927-1-colin.i.king@gmail.com


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