greybus: interface: Prefix hexadecimal values with '0x'

In order to clearly specify the base of values printed using sysfs
files, prefix hexadecimal values with '0x'. Also force the minimum width
(to be printed) for hexadecimal values to their sizes.

To make it more readable make the second argument to gb_interface_attr()
a proper string.

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>