[POWERPC] cpm2: Fix off-by-one error in setbrg().

The hardware adds one to the BRG value to get the divider, so it must
be subtracted by software.  Without this patch, characters will occasionally
be corrupted.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>