Powerpc 8xx CPM_UART maxidl should not depend on fifo size

maxidl register was set to fifo size. There is no reason to set this
register to same value as fifo size. Setting it now to 0x10 by default
as in the UCC UART driver.

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>