rtlwifi: rtl8192ee: Fix TX hang due to failure to update TX write point

Initially, the routine to update the write point in the FIFO buffer was
coded to save CPU time by not doing the calculation every interrupt. This
was an error and results in TX hangs.

Signed-off-by: Troy Tan <troy_tan@realsil.com.cn>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [V3.18]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>