vidcontrol: make size argument optional again for syscons

r273544 changed the -f option allow no arguments in vt mode (used to
reset the font back to the default), but broke the optionality of the
size argument for syscons. Drop the required argument from syscons'
optstring for -f so the optional argument handler works the same way
for both syscons and vt.

Reported by: bde
Sponsored by: The FreeBSD Foundation