PARISC pdc_console: fix bizarre panic on boot

upstream commit ef1afd4d

commit 721fdf34
Author: Kyle McMartin <kyle@shortfin.cabal.ca>
Date:   Thu Dec 6 09:32:15 2007 -0800

    [PARISC] print more than one character at a time for pdc console

introduced a subtle bug by accidentally removing the "static" from
iodc_dbuf. This resulted in, what appeared to be, a trap without
*current set to a task. Probably the result of a trap in real mode
while calling firmware.

Also do other misc clean ups. Since the only input from firmware is non
blocking, share iodc_dbuf between input and output, and spinlock the
only callers.

[jejb: fixed up rejections against the stable tree]

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>