Currently, the number of channels is set to 15 but in the case of DA8xx,
the number of channels is 4.
Update the driver to configure the number of channels at runtime.

Cc: stable@vger.kernel.org  # v4.12+
Signed-off-by: NAlexandre Bailon <abailon@baylibre.com>
Tested-by: NSekhar Nori <nsekhar@ti.com>
Signed-off-by: NBin Liu <b-liu@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>