rtl818x_pci: fix pci probe returns success when it fails

There are several exit path from the PCI probe function.
Some of them, that are taken in case of errors, forget to set the "err"
variable, that is returned by the probe function.
This can lead to the kernel thinking the probe function succeeds while it
didn't, and this in turn causes extra calls to the "remove" function.

This patch fix this problem by ensuring "err" variable is assigned to a proper
non-zero value in each exit path.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>