fbdev: via/via_clock: fix sparse warning

this patch fixes following sparse warning:

via_clock.c:33:12: warning: symbol 'via_slap' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>