spi: mchp-pci1xxx: Fix a possible null pointer dereference in pci1xxx_spi_probe

In function pci1xxxx_spi_probe, there is a potential null pointer that
may be caused by a failed memory allocation by the function devm_kzalloc.
Hence, a null pointer check needs to be added to prevent null pointer
dereferencing later in the code.

To fix this issue, spi_bus->spi_int[iter] should be checked. The memory
allocated by devm_kzalloc will be automatically released, so just directly
return -ENOMEM without worrying about memory leaks.

Fixes: 1cc0cbea

 ("spi: microchip: pci1xxxx: Add driver for SPI controller of PCI1XXXX PCIe switch")
Signed-off-by: Huai-Yuan Liu <qq810974084@gmail.com>
Link: https://msgid.link/r/20240403014221.969801-1-qq810974084@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>