usb: dwc2: remove set but unused variable

This patch removes a set but unused variable in hcd.c.

Fixes gcc warning:
variable ‘data_fifo’ set but not used [-Wunused-but-set-variable]

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Joshua Abraham <j.abraham1776@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>