net: microchip: Make FDMA config symbol invisible

There is no need to ask the user about enabling Microchip FDMA
functionality, as all drivers that use it select the FDMA symbol.
Hence make the symbol invisible, unless when compile-testing.

Fixes: 30e48a75 ("net: microchip: add FDMA library")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/8e2bcd8899c417a962b7ee3f75b29f35b25d7933.1727171879.git.geert+renesas@glider.be


Signed-off-by: Paolo Abeni <pabeni@redhat.com>