It currently assumes that there are always four channels, it would
cause the error if there is actually less than four channels. Change
that by getting number of channel from device tree.

For backwards-compatibility, it uses the default value (i.e. 4) when
there is no 'dma-channels' information in dts.
Signed-off-by: NZong Li <zong.li@sifive.com>
Acked-by: NPalmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Link: https://lore.kernel.org/r/f08a95b6582a51712c5b2c3cb859136d07bfa8b9.1648461096.git.zong.li@sifive.comSigned-off-by: NVinod Koul <vkoul@kernel.org>