drivers: sm7xxfb: Mark function as static in sm7xxfb.c

Mark function smtcfb_setmode() as static in sm7xxfb.c because it is not
used outside this file.

This eliminates the following warning in sm7xxfb.c:
drivers/staging/sm7xxfb/sm7xxfb.c:588:6: warning: no previous prototype for ‘smtcfb_setmode’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Javier M. Mellid <jmunhoz@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>