staging: sep: return -EFAULT on copy_from_user failure

copy_from_user() returns the number of bytes remaining to be copied,
and it fails if user pointer is invalid, so return -EFAULT
so that the user gets a FAULT.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>