USB: musb: respect usb_request->zero in control requests

In gadget mode the answer to a control request should be followed by
a zero-length packet if the amount transferred is an exact multiple of
the endpoint's packet size and the requests has its "zero" flag set.

This patch prevents the request from being immediately removed from the
queue when a control IN transfer ends on a full packet and "zero" is set.
The next time ep0_txstate is entered, a zero-length packet is queued and
the request is removed as fifo_count is 0.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>