hw/net/imx_fec: Remove unuseful FALLTHROUGH comments

hw/net/imx_fec: Remove unuseful FALLTHROUGH comments

We don't need to explicit these obvious switch fall through
comments. Stay consistent with the rest of the codebase.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191218192526.13845-6-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>