ath9k: Remove incorrect diversity initialization

Fast antenna diversity is required only for single chain
chips and the diversity initialization is done in the
per-family board setup routines. Enabling of diversity
should be done based on the calibrated EEPROM/OTP data,
doing it for all chips is incorrect.

Remove the code that sets the fast_div bit for all cards, since
the documentation for the AR_PHY_CCK_DETECT register says:

reg 642: sig_detect_cck
enable_ant_fast_div : Only used for single chain chips.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>