ACPI: APEI: EINJ: Fix less than zero comparison on a size_t variable

ACPI: APEI: EINJ: Fix less than zero comparison on a size_t variable

The check for c < 0 is always false because variable c is a size_t which
is not a signed type. Fix this by making c a ssize_t.

Fixes: 90711f7bdf76 ("ACPI: APEI: EINJ: Create debugfs files to enter device id and syndrome")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20250624201032.522168-1-colin.i.king@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>