rt2x00pci: Disable memory-write-invalidate when the driver exits

The driver calls pci_set_mwi to enable memory-write-invalidate when it
is initialized, but does not call pci_clear_mwi when it is removed. Many
other drivers calls pci_clear_mwi when pci_set_mwi is called, such as
r8169, 8139cp and e1000.

This patch adds pci_clear_mwi in error handling and removal procedure,
which can fix the problem.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>