usb: xhci: Add support for zero length packet

usb protocol requires host to send zero length packet (zlp) if the
out going packet length is multiple of usb max packet size. However,
this is determined by application level protocol and application
drivers set ZERO_PACKET flag if zlp needs to be supported. Add
support to check this flag and send the zlp whenever out transfer
length is multiple of usb max packet size.

Change-Id: Ic1b67ab017dd0663cec7d9ee4dd489d8193da957
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>