[POWERPC] Fix compile failure in cpm_uart_cpm2

Fix bug that exists in kernel.org since 2.6.17rc4 - compiles fail if
CONFIG_SERIAL_CPM_SMC is defined.  Tested on a board using SMC1 console.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>