dz: always check if it is safe to console_putchar()

Polled transmission is tricky enough with the DZ11 design.  While "loop" is
set to a high value, conceptually you are not allowed to transmit without
checking whether the device offers the right transmission line (yes, it is the
device that selects the line -- the driver has no control over it other than
disabling the transmitter offered if it is the wrong one), so the loop has to
be run at least once.

Well, the '1977 or PDP11 view of how serial lines should be handled...  Except
that the serial interface used to be quite an impressive board back then
rather than chip.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>