vt_vga: Give only the character part of term_char_t to vga_get_cp437()

This fixes a bug where vga_get_cp437() was called with an invalid
argument. The screen was then filled with '?' instead of the actual
character.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270388 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f