iwlwifi: pcie: fix SPLC structure parsing

The SPLC data parsing is too restrictive and was not trying find the
correct element for WiFi.  This causes problems with some BIOSes where
the SPLC method exists, but doesn't have a WiFi entry on the first
element of the list.  The domain type values are also incorrect
according to the specification.

Fix this by complying with the actual specification.

Additionally, replace all occurrences of SPLX to SPLC, since SPLX is
only a structure internal to the ACPI tables, and may not even exist.

Fixes: bcb079a1 ("iwlwifi: pcie: retrieve and parse ACPI power limitations")
Reported-by: Chris Rorvick <chris@rorvick.com>
Tested-by: Paul Bolle <pebolle@tiscali.nl>
Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>