hpet: Make cmd parameter of hpet_ioctl_common() unsigned

The value passed by the two callers of the function is unsigned anyway.

Making the parameter unsigned fixes the following warning when building
with clang:

drivers/char/hpet.c:588:7: error: overflow converting case value to switch condition type (2149083139 to 18446744071563667459) [-Werror,-Wswitch]
        case HPET_INFO:
             ^
include/uapi/linux/hpet.h:18:19: note: expanded from macro 'HPET_INFO'
                        ^
include/uapi/asm-generic/ioctl.h:77:28: note: expanded from macro '_IOR'
                                ^
include/uapi/asm-generic/ioctl.h:66:2: note: expanded from macro '_IOC'
        (((dir)  << _IOC_DIRSHIFT) | \

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>