ACPI: asus_acpi: rework model detection

This patch reworks laptop model detection.

This addresses the Samsung P30 issue, where the INIT method would return no
object, but the implicit return in the AML interpreter would confuse the
driver. It also accounts for a newer batch of Asus models whose INIT
returns ACPI_TYPE_BUFFER instead of STRING.

The handling is now much leaner, if we get a buffer or a string, we check
against known values, in every other case we use a different path
(currently DSDT signatures). The bulk of this patch is separating the
string matching from asus_hotk_get_info() into a separate function.

This patch properly fixes http://bugme.osdl.org/show_bug.cgi?id=5067 and
http://bugme.osdl.org/show_bug.cgi?id=5092

 and makes the driver fully
functional again with acpi=strict on all machines.

Signed-off-by: Karol Kozimor <sziwan@hell.org.pl>
Signed-off-by: Len Brown <len.brown@intel.com>