prism54: set carrier flags correctly

> prism54 should set the carrier flags correctly when it thinks the
> link can be used.

Agreed, so sure, this is OK but I rather we turn the carrier on
or off *before* sending an event, like this.

Signed-off-by: Roy Marples <uberlord@gentoo.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>