[POWERPC] of_serial: Fix possible null dereference.

The of_serial driver queries the current-speed property and attempts
to use it to register the custom_divisor property of the uart_port.
However, if current-speed is not set, then this code will dereference
a bad pointer.  The fix is to only set custom_divisor when a
current-speed property appears in the device tree.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>