bcm47xx: enable adm6996 only on legacy subtarget

bcm47xx: enable adm6996 only on legacy subtarget

The generic subtarget supports only a few devices. None of
these devices are equipped with a ADM6996 switch. On the
mips74k subtarget, the driver for the adm6996 switch is
disabled. So it seems that the ADM6996 driver should
be enabled only on the legacy subtarget.

Support for ADM6996 switches was enabled in commit 68081fc1c8c7814b1c064431eb2e364f9226801a.
At the time when this driver was enabled the bcm47xx
target had only one subtarget.

Switches used by individual devices suported by the generic
subtarget are listed below.

Device			Switch
Edimax PS-1208MFG	int. SoC
Linksys WRT300N v1.1	Broadcom BCM5325
Linksys WRT310N v1	Broadcom BCM5397
Linksys WRT350N v1	Broadcom BCM5397
Linksys WRT610N v1	Broadcom BCM53115
Linksys WRT610N v2	Broadcom BCM53115
Linksys E3000 v1	Broadcom BCM53115

Reduce uncompressed kernel size by 8320 Bytes.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>