USB: host: ohci-at91: fix non static symbol warning

Fixes the following sparse warning:

drivers/usb/host/ohci-at91.c:141:15: warning:
 symbol 'at91_dt_syscon_sfr' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>