HID: wacom: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/hid/wacom_wac.c: In function ‘wacom_setup_pen_input_capabilities’:
drivers/hid/wacom_wac.c:3506:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   __clear_bit(ABS_MISC, input_dev->absbit);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/hid/wacom_wac.c:3508:2: note: here
  case WACOM_MO:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>