can: ucan: fix non-atomic allocation in completion handler

USB completion handlers are called in atomic context and must
specifically not allocate memory using GFP_KERNEL.

Fixes: 9f2d3eae ("can: ucan: add driver for Theobroma Systems UCAN devices")
Cc: stable <stable@vger.kernel.org>     # 4.19
Cc: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Cc: Martin Elshuber <martin.elshuber@theobroma-systems.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>