serial: st-asc: Don't call BUG in asc_console_setup()

In order to prevent an asc instance to be used as early console, BUG_ON is
used on either mapbase or membase being NULL.

Problem is that this condition is also true when we set console to be a ttyASx
different to the first asc instance being probed.

Instead of calling BUG_ON, it now returns -ENXIO when either mapbase or
membase is NULL.

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>