Input: elantech - annotate fall-through case in elantech_use_host_notify()

This avoids a warning when building with -Wimplicit-fallthrough.

Fixes: 883a2a80

 ("Input: elantech - enable SMBus on new (2018+) systems")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>