mediatek: mt7623: disable swconfig

mediatek: mt7623: disable swconfig

The mt7623 subtarget supports 2 devices:
 * Bananapi BPi-R2 (added in 1f068588efddf0175e954ffc07ec8478bddd52c7, 7762c07c88980cff85ec20c12f18cd172260e9d9),
 * UniElec U7623-02 (added in 4def81f30f2e89eeecc6ec06e3091f1dd6b3c3d9).

Both devices support DSA from the beginning, thus
swconfig can be safely disabled.

In the past, the subtarget mt7623 also supported
the mt7623 reference board. This board originally
supported swconfig, and was later converted to DSA
(64175ffb7911f5fcd3907df09fdb9bfab53ee641) and then dropped (1ab81bf02d750c9a2a447dbf4fdb323921f291d7).

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