platform/x86: thinkpad_acpi: Switch to using sparse-keymap helpers

Switch the hotkey keymap handling over to the sparse-keymap helpers,
there should be no functional changes from this.

Note all the mappings to KEY_UNKNOWN are removed since that is the default
behavior of sparse_keymap_report_event() for unknown scancodes.

Also drop the big comment about making changes to the keymaps since
the contents of that comment are mostly obsolete.

Tested-by: Mark Pearson <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>
Reviewed-by: Mark Pearson <[email protected]>
Reviewed-by: Ilpo Järvinen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: jwrdegoede