Bluetooth: btnxpuart: Add an error message if FW dump trigger fails

This prints an error message if the FW Dump trigger command fails. This
scenario is mainly observed in legacy chipsets 8987 and 8997 and also
IW416, where this feature is unavailable due to memory constraints.

Fixes: 998e447f ("Bluetooth: btnxpuart: Add support for HCI coredump feature")
Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>