staging: rtl8712: fixed multiple line derefence issue

Checkpatch emits WARNING: Avoid multiple line dereference.

Checkpatch warning is fixed by:
* Trivial indentation improvement and
* Using += instead of + helps shortening the statement.

Signed-off-by: Prasant Jalan <prasant.jalan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>