toshiba_acpi: Adapt /proc/acpi/toshiba/keys to TOS1900 devices

Since the introduction of TOS1900 devices support to the driver, the
"keys" entry under the proc directory was broken, given that it only
handled TOS620X devices accordingly.

This patch adapts the code to show the hotkey values of TOS1900
devices too, and in case some programs are still using that interface,
hotkeys reporting should now work on these devices.

Signed-off-by: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>