Add support for "ethtool -i" to prism54 driver.

ethtool -i queries the specified device for
associated driver information.

This helps tools like Fedora's system-config-network to
provide GUI management of network devices.
I learned how to write this patch by reading the ipw2100
driver code.
Signed-off-by: Kai Engert <kengert@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>