bridge: inherit slave devices needed_headroom

Some slave devices may have set a dev->needed_headroom value which is
different than the default one, most likely in order to prepend a
hardware descriptor in front of the Ethernet frame to send. Whenever a
new slave is added to a bridge, ensure that we update the
needed_headroom value accordingly to account for the slave
needed_headroom value.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: fd094808a06e290432fc13c09aae808aea34d2ca
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I98ecb7c53b640331c364cd5886913d26af1b7781
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>