vme: add vme_init_bridge for common bridge init

Consolidate vme_bridge structure setup that every bridge was required
to do itself. This came about because .irq_mtx is only used within the
VME core, but was required to be setup externally.

This returns the structure passed in to support shorthand like this:

    bridge = vme_init_bridge(&priv->bridge);

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Acked-by: Martyn Welch <martyn@welchs.me.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>