USB: fsl_qe_udc: Fix stalled TX requests bug

While disabling an endpoint the driver nuking any pending requests,
thus completing them with -ESHUTDOWN status. But the driver doesn't
clear the tx_req, which means that a next TX request (after
ep_enable), might get stalled, since the driver won't queue the new
reqests.

This patch fixes a bug I'm observing with ethernet gadget while
playing with ifconfig usb0 up/down (the up/down sequence disables
and enables `in' and `out' endpoints).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>