HID: logitech-hidpp: do not return the name length

We do not make any use of the actual name length get through
hidpp_get_device_name(). Original patch by Benjamin Tissoires, this
patch also replaces a (now) unnecessary goto by return NULL.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>