usb/early: remove set but not used variable 'remain_length'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/usb/early/xhci-dbc.c: In function 'xdbc_handle_tx_event':
drivers/usb/early/xhci-dbc.c:720:9: warning:
 variable 'remain_length' set but not used [-Wunused-but-set-variable]

It never be used since introduction in
commit aeb9dd1d ("usb/early: Add driver for xhci debug capability")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>