USB: EHCI: Add support for running URB giveback in tasklet context

commit 428aac8a added changes
to release ehci->lock during URB giveback. If HCD_BH is set for
HC driver's flags, URB giveback will be done in tasklet context
instead of interrupt context, so the ehci->lock needn't to be
released any more before calling usb_hcd_giveback_urb(). Hence
add HCD_BH flag in controller driver to support this.

Change-Id: Ia80caa0d92e61cea2ccf7076db2c0dc676dd5e29
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>