ukbd(4): Push LED events in ioctl handler rather than in xfer callback

If LED state is set through evdev interface, than asynchronous nature
of USB transfer callback can lead to change of order of events echoed
back to userland as it causes LED events to be echoed with some lag.

Fix that with echoing of LED events synchronously in ioctl handler.

Reviewed by: hselasky
Obtained from: sysutils/iichid
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D27750