power: reset: Fix handling of OEM commands

The change to remove the native MSM reboot driver
incorrectly refactored the OEM command handling code to use
kstrtoul() to parse the OEM command ID. Fix that.

CRs-Fixed: 663628
Change-Id: I073043266127818815fb4e6376c92c2b2fd953a5
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>