USB: cdc-acm: ignore Exar XR21V141X when serial driver is built

The Exar XR21V141X can be used in either ACM mode using the cdc-acm
driver or in "custom driver" mode in which further features such as
hardware and software flow control, GPIO control and in-band line-status
reporting are available.

In ACM mode the device always enables RTS/CTS flow control, something
which could prevent transmission in case the CTS input isn't wired up
correctly.

Ensure that cdc_acm will not bind to the device if the custom USB-serial
driver is enabled.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/20201122170822.21715-4-mani@kernel.org


[ johan: rewrite commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>