mmc: core: prepend 0x to OCR entry in sysfs

The sysfs entry "ocr" was missing the 0x prefix to identify it as hex
formatted.

Fixes: 5fb06af7 ("mmc: core: Extend sysfs with OCR register")
Signed-off-by: Bastian Stender <bst@pengutronix.de>
Cc: <stable@vger.kernel.org> # v4.8+
[Ulf: Amended change to also cover SD-cards]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>