spi: mpc5xxx-psc: Fix unsigned expression compared with zero

There is two warnings reported by coccinelle:

./drivers/spi/spi-mpc512x-psc.c:493:5-13: WARNING:
	Unsigned expression compared with zero: mps -> irq     <     0
./drivers/spi/spi-mpc52xx-psc.c:332:5-13: WARNING:
	Unsigned expression compared with zero: mps -> irq     <     0

The commit "208ee586"
("spi: mpc5xxx-psc: Return immediately if IRQ resource is unavailable")
was to check whether the IRQ resource is unavailable. When the IRQ
resource is unavailable, an error code is returned, however, the type
of "mps->irq" is "unsigned int", causing the error code to flip. Modify
the type of "mps->irq" to solve this problem.

Fixes: 208ee586

 ("spi: mpc5xxx-psc: Return immediately if IRQ resource is unavailable")
Signed-off-by: Li Zetao <lizetao1@huawei.com>
Link: https://lore.kernel.org/r/20230803134805.1037251-1-lizetao1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>