bridge link: add option 'self'

bridge link: add option 'self'

Currently self is set internally only if hwmode is set.
This makes it necessary for the hw to have a mode.
There is no hwmode really required to go to hardware. So, introduce
self for anybody who wants to target hardware.

v1 -> v2
    - fix a few bugs. Initialize flags to zero: this was required to
    keep the current behaviour unchanged.

v2 -> v3
    - fix comment

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>