misc/pvpanic: fix return value check in pvpanic_pci_probe()

In case of error, the function pci_iomap() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Link: https://lore.kernel.org/r/20210330013659.916-1-linqiheng@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>