misc/byteorder.h: Mark bswap_16 & bswap_32 as internal helpers

These shouldn't be exported in the official doxygen documentation
since they should only be used through the other conversion APIs.

Change-Id: I75880b42892cbfce769192ec2e8c296c954979bd
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>