usb: xhci: dbc: remove redundant pointer dbc

Pointer dbc is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
variable 'dbc' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>