ACPI / sysfs: Treat the count field of counter_show() as unsigned

The count field is an unsigned 32bit value, and the
counter_show() function should also treat it as a unsigned
value.

Otherwise the counter may show negative number as we found on a
machine:
...
gpe23:        0   invalid
gpe24: -2071733   enabled
gpe25:        0   invalid
...
gpe_all: -2070980
sci: -2070949

Signed-off-by: Nan Li <nli@suse.com>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>