staging: rts5139: Use correct USB transfer interval to fix syslog spamming

Using correct transfer interval as specified by the USB endpoint
when doing the interrupt transfer fixes the warning printed by
xhci USB core on every transfer that resulted in spamming
"xhci_queue_intr_tx: 74 callbacks suppressed" to syslog
every 5 seconds.

Signed-off-by: Marcus Overhagen <marcus.overhagen@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>