HID: core: add two new usages for digitizer

On Feb 17, 2014, two new usages are approved to HID usage Table 18 -
Digitizer Page:

5A	Secondary Barrel Switch		MC	16.4
5B	Transducer Serial Number	SV	16.3.1

This patch adds relevant definitions to hid/input. It also removes
outdated comments in hid.h.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>