platform/x86: thinkpad_acpi: Initialize local in_tablet_mode and type

linux-next reported in_tablet_mode and type may be used uninitialized
after:

b3180028

 ("platform/x86: thinkpad_acpi: Move tablet detection into separate function")

This turns out to be a false positive as the pr_info call cannot be
reached if tp_features.hotkey_tablet (global scope) is 0, and
in_tablet_mode and type are assigned in both places
tp_features.hotkey_tablet is assigned.

Regardless, to make it explicit and avoid further reports, initialize
in_tablet_mode to 0 and type to NULL.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Lyude <lyude@redhat.com>