platform/x86/amd/pmf: Fix unsigned comparison with less than zero

The return value from the call to amd_pmf_get_pprof_modes() is int.
However, the return value is being assigned to an unsigned char
variable 'mode', so making 'mode' an int.

silence the warning:
./drivers/platform/x86/amd/pmf/sps.c:183:5-9: WARNING: Unsigned expression compared with zero: mode < 0

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5995
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230727014315.51375-1-yang.lee@linux.alibaba.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>