ARM: OMAP2+: nand: fix build error when CONFIG_MTD_ONENAND_OMAP2=n

commit 8259573b (ARM: OMAP2+: nand: Make board_onenand_init() visible
to board code) broke the build for configs with OneNAND disabled.  By
removing the static in the header file, it created a duplicate definition
in the .c and the .h files, resuling in a build error:

/work/kernel/omap/dev/arch/arm/mach-omap2/board-flash.c:102:111: error: redefinition of 'board_onenand_init'
/work/kernel/omap/dev/arch/arm/mach-omap2/board-flash.h:56:51: note: previous definition of 'board_onenand_init' was here
make[2]: *** [arch/arm/mach-omap2/board-flash.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [arch/arm/mach-omap2] Error 2
make: *** [sub-make] Error 2

Fix this by removing the duplicate dummy entry from the C file.

Cc: Enric Balletbò i Serra <eballetbo@gmail.com>
Cc: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>