rtlwifi: rtl8192ee: Fix problems with calculating free space in FIFO

rtlwifi: rtl8192ee: Fix problems with calculating free space in FIFO

commit 6d4beca3775222884e1ee9d48ef586c438c3dfa1 upstream.

This driver utilizes a FIFO buffer for RX descriptors. There are four places
in the code where it calculates the number of free slots. Several of those
locations do the calculation incorrectly. To fix these and to prevent future
mistakes, a common inline routine is created.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>