Fix smatch warning:

drivers/soc/fsl/qe/ucc.c:526
 ucc_set_tdm_rxtx_clk() warn: unsigned 'tdm_num' is never less than zero.

'tdm_num' is u32 type, never less than zero.
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>