rtlwifi: Store loop index in local variable

_rtl_pci_rx_interrupt uses rtlpci->rx_ring[rx_queue_idx].idx a
few times, so store it in a separate variable.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>