usb: musb: remove unused pipe variable

This patch fix the following build warning:
drivers/usb/musb/musb_host.c:1809:8: warning: variable 'pipe' set but not used [-Wunused-but-set-variable]
by removing the pipe variable in musb_host_rx()

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>