[media] arm: omap2: rx51-peripherals: fix build warning

commit 68a3c043 ([media] ARM: OMAP2: RX-51: update
si4713 platform data) updated board-rx51-peripherals.c
so that si4713 could be easily used on DT boot, but
it ended up introducing a build warning whenever
si4713 isn't enabled.

This patches fixes that warning:

arch/arm/mach-omap2/board-rx51-peripherals.c:1000:36: warning: \
	‘rx51_si4713_platform_data’ defined but not used [-Wunused-variable]
 static struct si4713_platform_data rx51_si4713_platform_data = {

Cc: Sebastian Reichel <sre@kernel.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>