toshiba_acpi.c: Add key_entry for a lone FN keypress

A lone FN key press on a Toshiba Portégé R700 without another key in
conjunction results in an ACPI event and a spurious error message on
the console.

Add a key entry to map this event to a KEY_FN keypress. This prevents
the console message.

Signed-off-by: Jon Dowland <jmtd@debian.org>
Signed-off-by: Matthew Garrett <mjg@redhat.com>