usb: musb: remove unused frame variable

This patch fix the following warning
drivers/usb/musb/musb_host.c:223:8: warning: variable 'frame' set but not used [-Wunused-but-set-variable]
by remove the frame variable in musb_start_urb().

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>