usb: musb: musb_cppi41: Configure the number of channels for DA8xx

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: Alexandre Bailon <abailon@baylibre.com>
Tested-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>