The DWC I2S driver treats the channel count register values as if they
encode a power of two (2, 4, 8, 16), but they actually encode a
multiple of 2 (2, 4, 6, 8).
Also improve the error message when asked for an unsupported number
of channels.
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
d5f9c4607f