commit 6d4beca3 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>