omap3: igep3: make igep3_flash_init static

igep3_flash_init is not used beyond the scope of the file, make it
static instead.

Fixes sparse warning:
arch/arm/mach-omap2/board-igep0030.c:106:13: warning: symbol 'igep3_flash_init' was not declared. Should it be static?

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