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

In the Linux kernel, the following vulnerability has been resolved:

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.