usbnet: fix memory leak in error case

usbnet_write_cmd_async() mixed up which buffers
need to be freed in which error case.

v2: add Fixes tag
v3: fix uninitialized buf pointer

Fixes: 877bd862

 ("usbnet: introduce usbnet 3 command helpers")
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20220705125351.17309-1-oneukum@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>