staging: Mediatek: Use individual config flags in Makefile

These drivers can be useful on other MT76xx SoCs, which have compatible
peripherals.  The drivers were selectable in Kconfig, but they were
quietly excluded from the build because the SOC_MT7621 chip was not
selected.  So, make the Makefiles use the same flags as Kconfig for
these drivers.

mt7621-dma and mt7621-dts are left alone because they truly do require
that SoC.

I have personally confirmed that the mt7621-spi driver works on the
MT7688, which was what prompted this change.

Cc: linux-kernel@vger.kernel.org
Cc: devel@driverdev.osuosl.org
Cc: NeilBrown <neil@brown.name>
Cc: sergio.paracuellos@gmail.com
Signed-off-by: George Hilliard <thirtythreeforty@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>