fs: Use filesystem methods instead of switch()

We can use the available methods and avoid using switch(). When the
filesystem is not supported, we fall through to the 'unsupported'
methods: fs_probe_unsupported() prints an error, so the others do
not need to.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>