Remove every trace of SERIAL_MAGIC

Remove every trace of SERIAL_MAGIC

This means removing support for checking magic in amiserial.c
(SERIAL_PARANOIA_CHECK option), which was checking a magic field which
doesn't currently exist in the struct. That code hasn't built at least
since git.

Removing the definition from the header is safe anyway as that code was
from another driver and not including it.

Signed-off-by: Pascal Terjan <pterjan@google.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20191105192749.67533-1-pterjan@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>