mei: mei_write: revamp error path handling

1. unify common amt and regular error path and use it early in the
  function
2. fix indentation
3. propagate error code directly from copy_from_user
4. print out errors using dev_err instead of dev_dbg

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>