mgmt/mcumgr/lib: Make version string work with CONFIG_NEWLIB_LIBC

By default the newlib does not compile with %hhu processing,
to avoid turning on additional options for newlib compilation
this commit changes formatting string to use %hu instead.
The commit fixes problem with malformed version string being
returned, by smp_svr, when CONFIG_NEWLIB_LIBC=y is set together
with CONFIG_NEWLIB_LIBC_NANO=y.

Fixes #45261

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>