rtl8xxxu: Use flag to indicate whether device has TX report timer support

Use a fileops flag to indicate whether the device has TX report timer
support. This will make it easier to include future devices such as
8188eu to use the TX report timer.

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