rtlwifi: Fix incorrect return of IRQ_HANDLED

The recent discussion regarding an incorrect return of IRQ_HANDLED
from rt2800pci caused me to look at this PCI interrupt routine. I
discovered that changes were needed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>