vt/fbcon: update scrl_erase_char after 256/512-glyph font switch

Addendum to commit c9e587ab

 ("vt: fix
background color on line feed").

vc->vc_scrl_erase_char was not updated when fbcon switches between
256- and 512-glyph fonts.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>