greybus: firmware: convert to bundle driver

Convert the legacy firmware protocol driver to a bundle driver.

This also fixes a potential crash should a (malicious) module have sent
an early request before the private data had been initialised.

Note that the firmware protocol needs to support the version request
indefinitely since it has been burnt into ROM.

In order to avoid having to update current module-loading scripts, keep
this driver internal to greybus core at least until modalias support is
added.

Note that there is no MODULE_DEVICE_TABLE defined for firmware as we
cannot have two greybus tables in one module on ancient 3.10 kernels and
that the legacy driver is currently also internal to core. This needs be
added once the driver can be built as a module.

Testing Done: Tested on DB3.

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