Staging: comedi: Return -EFAULT if copy_to_user() fails

Return -EFAULT on error instead of the number of bytes remaining to be
copied.

Fixes: bac42fb2 ("comedi: get rid of compat_alloc_user_space() mess in COMEDI_CMD{,TEST} compat")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/X8c3pfwFy2jpy4BP@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>