arm: omap2: mux: fix compile warning

Commit 8419fdba
(omap2+: Add omap_mux_get_by_name) introduced the following
compile warning:

arch/arm/mach-omap2/mux.c: In function '_omap_mux_get_by_name':
arch/arm/mach-omap2/mux.c:163:17: warning: 'found_mode' may be used
uninitialized in this function

Signed-off-by: Felipe Balbi <balbi@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>