greybus: Prefix hexadecimal values with 0x while printing them

To clearly specify the base for printed values, prefix hexadecimal
values with 0x.

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