ath5k: Fix fast channel switching

Fast channel change fixes:

a) Always set OFDM timings
b) Don't re-activate PHY
c) Enable only NF calibration, not AGC

https://bugzilla.kernel.org/show_bug.cgi?id=27382



Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>