USB: at91: fix clk_get error handling

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Cc: linux-usb@vger.kernel.org