Several of these drivers have there TX randomly blocked for 3~5 seconds while
measuring tx throughput (iperf). The root couse happens in rtl_pci_flush().
The function uses a while-loop to wait for TX queue length to decrease to 0.
The TX queue length counts the number of packets that are queued in the driver.
The driver relys on the TX OK interrupt to return skb and reduce TX queue length.

The interrupt subroutine disables interupts, reads the interrupt registers, and
then clears the registers in the beginning of _rtl_pci_interrupt(). After all
interupts process are finished, the driver invokes enable_interrupt() to enable
interupts. This behavior is normal for an interrupt subroutine.

But enable_interrupt() invokes clear_interrupt() again. This unexpected interrupt
clearing may cleari me fresh TX OK interrupts. These missing interrupts cause TX
queue length to never reduce to 0i, which causes rtl_pci_flush() to be stuck in
unterminated while-loop.

This patch removes clear_interrupt() in enable_interrupt() to avoid this behavior.
Signed-off-by: Vincent Fann <vincent_fann@realtek.com>
Signed-off-by: Shao Fu <shaofu@realtek.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [3.18+]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>