atm: clean up a put_user() calls

Unlike copy_from_user(), put_user() and get_user() return -EFAULT on
error.  Use the error code directly instead of setting it.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/04a018e8-7433-4f67-8ddd-9357a0114f87@moroto.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>