greybus: uart: Kill reference_count hack

This was done long back and was probably the best bet then, but it looks
really bad to have it this way now.

Kill the hack and implement proper driver init()/exit() routines to do
the same thing.

Tested using gbsim by hotplugging uart manifest and removing it later.
Also tried removing the gb-phy module to test the exit path properly.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>