rtl818x: pci_iomap() should pair with pci_iounmap()

Currently the driver uses pci_iomap() but iounmap() is called in
the error path

Change to use pci_iounmap() instead.

Reported-by: Huqiu Liu <liuhq11@mails.tsinghua.edu.cn>
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>