spi: microchip-core: fix potentially incorrect return from probe

If platform_get_irqi() returns 0, the error case will be triggered but
probe() will return 0 rather than an error. Ape the other drivers using
this pattern and return -ENXIO.

Reported-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/linux-spi/20220609055533.95866-2-yang.lee@linux.alibaba.com/


Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Fixes: 9ac8d176 ("spi: add support for microchip fpga spi controllers")
Link: https://lore.kernel.org/r/20220614065809.1969177-1-conor.dooley@microchip.com


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