Remove the initial NF completion check.

This is taking quite a while for some people in some situations
(eg AR5418 in phk's Abusive Radio Environment).

Instead, the rest of the calibration related code should
ensure that a NF calibration has occured before reading NF
values and kicking off another NF calibration.

The channel should also likely be marked as "noisy" (CWINT)
if the NF calibration takes too long.