ARM: OMAP2+: remove superfluous NULL pointer check

The NULL pointer check for superset->muxnames will always evaluate
true since muxnames is an array within struct omap_mux. Remove the
superfluous check to avoid warnings when using LLVM/clang.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>