Update Kconfig with enhanced help text for hid-plantronics driver.

Update hid-plantronics.c to identify device type and correctly map
either the vendor unique or consumer control volume up/down usages
to KEY_VOLUMEUP and KEY_VOLUMEDOWN events. Unmapped usages are ignored
to prevent core mapping of unknown usages to random mouse events.

Tested on ChromeBox/ChromeBook with various Plantronics devices.
Signed-off-by: Terry Junge <terry.junge@plantronics.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>