cpm_uart: Fix cpm uart corruption with PREEMPT_RT

Fix CPM serial port corruption when running with CONFIG_PREEMPT_RT.
Userland usage of console, and kernel printf's were stepping on each others toes.
Also only take lock if not in an oops.

Signed-off-by: Rune Torgersen <runet@innovsys.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>