video: ipu: simplify if else code

Simplify if/else code, since if channel equals to MEM_BG_SYNC or
MEM_FG_SYNC, we have value 5 for 'dc_chan'.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>