Disable TX IQ calibration, it was prematurely enabled in
previous versions.

Cc: Paul Shaw <Paul.Shaw@Atheros.com>
Cc: Thomas Hammel <Thomas.Hammel@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>