Powerpc 8xx CPM_UART desynchronisation

This patch fixes a desynchronisation problem with CPM UART driver on
Powerpc MPC8xx. The problem happens if data is received before the device
is open by the user application.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>