USB: storage: fix compile warning

This patch should fix the below compile warning:

drivers/usb/storage/protocol.c: In function 'usb_stor_access_xfer_buf':
drivers/usb/storage/protocol.c:155:22: warning: comparison of distinct
pointer types lacks a cast [enabled by default]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>