When calculating ufsi, the function was relying
on the slot processed by the unicast fhss timer
callback, which can be delayed. When it happens
the slot value is wrong, and the ufsi is incorrect.

The ufsi is then used by the peer to determined
the reply channel, so the devices are thus
unsynchronized until the next uplink packet.

Co-authored-by: Jarkko Paso <jarkko.paso@arm.com>