staging: wfx: fix copy_{to,from}_user() usage

staging: wfx: fix copy_{to,from}_user() usage

On error, copy_to_user() returns number of bytes remaining. Driver
should return -EFAULT.

Fixes: 4f8b7fabb15d ("staging: wfx: allow to send commands to chip")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191008094232.10014-6-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>