greybus: kernel_ver.h: define U32_MAX and U64_MAX

These were not defined, and I just posted patches that use them.

Signed-off-by: Alex Elder <elder@linaro.org>
Tested-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>