rtl8xxxu: Stop log spam from each successful interrupt

As soon as debugging is turned on, the logs are filled with messages
reporting the interrupt status. As this quantity is usually zero, this
output is not needed. In fact, there will be a report if the status is
not zero, thus the debug line in question could probably be deleted.
Rather than taking that action, I have changed it to only be printed
when the newly added RTL8XXXU_DEBUG_INTERRUPT bit is set in the debug
mask.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>