net: wan: slic_ds26522: Allow driver to built if COMPILE_TEST is enabled

The driver only has runtime but no build time dependency with FSL_SOC ||
ARCH_MXC || ARCH_LAYERSCAPE.  So it can be built for testing purposes if
the COMPILE_TEST option is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>