ukbd(4): Do not serialize evdev key events

Unlike AT keyboards, HID devices are able to send all pc105 key
states within a single report. Let evdev to transmit all key state
changes within a single report too.

Reviewed by: hselasky
Obtained from: sysutils/iichid
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D27749

(cherry picked from commit 769935a4edf8a75805000900d44a66ad5d2eaabe)