[SERIAL] 8250: add locking to console write function

x86 SMP breaks as a result of the previous change, we have no real
option other than to add locking to the 8250 console write function.
If an oops is in progress, try to acquire the lock.  If we fail to
do so, continue anyway.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>