ath5k: Add a module parameter to disable hw rf kill switch

Add a module parameter to disable hw rf kill switch (GPIO interrupt) because
in some cases when the card doesn't come with the laptop, EEPROM configuration
doesn't match laptop's configuration and rf kill interrupt always fires up and
disables hw. I thought of moving this to debugfs and make it per-card but
this way it's easier for users and distros to handle.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>