spi: ppc4xx: handle irq_of_parse_and_map() errors

Zero and negative number is not a valid IRQ for in-kernel code and the
irq_of_parse_and_map() function returns zero on error.  So this check for
valid IRQs should only accept values > 0.

Fixes: 44dab88e ("spi: add spi_ppc4xx driver")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://patch.msgid.link/20240722141822.1052370-1-make24@iscas.ac.cn


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