omap2+: wdt: trivial sparse fixes

omap2_wd_timer_disable is declared in wdtimer.h and used by hwmod
function pointers for usage, the header inclusion is necessary
to ensure that the prototype and function remains consistent.
omap_wdt_latency is passed as a pointer and does not need global scope

Fixes sparse warnings:
arch/arm/mach-omap2/devices.c:981:31: warning: symbol 'omap_wdt_latency' was not declared. Should it be static?
arch/arm/mach-omap2/wd_timer.c:27:5: warning: symbol 'omap2_wd_timer_disable' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>