greybus: hd: rename host-driver structure

Rename host-driver structure to gb_hd_driver to more clearly separate it
from the host-device structure.

Also remove an outdated description of the struct.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>