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

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

Fixes: 46bc5c40 ("mmc: core: Export device lifetime information through sysfs")
Signed-off-by: Bastian Stender <bst@pengutronix.de>
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>