ath5k: update AR5K_PHY_RESTART_DIV_GC values to match masks

#define AR5K_PHY_RESTART_DIV_GC               0x001c0000
is 3 bit wide.

The previous values of 0xc and 0x8 are 4bit wide and bigger than the mask.

Writing 0 and 1 to AR5K_PHY_RESTART_DIV_GC is consistent with the comments and
initvals we have in the HAL.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>