mac80211: rt2x00: prune more unnecessary exports

mac80211: rt2x00: prune more unnecessary exports

The calibration patches for MT7620 unnecessarily export symbols and
populate never accessed function pointers. Remove all that and make
functions static as the only place where each of those functions is
called is within rt2800lib.c.
Also make code more readable by fixing indentation, removing
unnecessary parantheses and simplifying some instructions using
shorthands here and there.

Signed-off-by: Daniel Golle <[email protected]>