HID: usbhid: extend the polling interval configuration to keyboards

For mouse and joystick devices user can change the polling interval
via usbhid.mousepoll and usbhid.jspoll.
Implement the same thing for keyboards, so user can
reduce(or increase) input latency this way.

This has been tested with a Cooler Master Devastator with
kbpoll=32, resulting in delay between events of 32 ms(values were taken
from evtest).

Signed-off-by: Filip Alac <filipalac@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>