ath5k: Disable fast channel switching by default

Disable fast channel change by default on AR2413/AR5413 due to
some bug reports (it still works for me but it's better to be safe).
Add a module parameter "fastchanswitch" in case anyone wants to enable
it and play with it.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>