tty/vt: don't set font mappings on vc not supporting this

tty/vt: don't set font mappings on vc not supporting this

commit 9e326f78713a4421fe11afc2ddeac07698fac131 upstream.

We can call this function for a dummy console that doesn't support
setting the font mapping, which will result in a null ptr BUG. So check
for this case and return error for consoles w/o font mapping support.

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=59321
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: this function doesn't take a lock, so doesn't
 need to unlock on error]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>