motu: when 1 or 2 pad bytes are required to satisfy the data alignment requirements...

motu: when 1 or 2 pad bytes are required to satisfy the data alignment requirements of the interface, ensure they are zeroed before transmission (consistent with the behaviour of MOTU drivers under other systems).  Being less than the sample size (3 bytes), these bytes cannot be associated with any ports and therefore can take on non-zero values under certain conditions.  By explicitly zeroing them it is ensured that only pad bytes of zero are ever sent to the interface.  This issue was discovered while investigating the on-going failure to get playback working on the 828mk1.  Unfortunately this patch has had no effect 828mk1 playback.

git-svn-id: svn+ssh://ffado.org/ffado/trunk@2370 2be59082-3212-0410-8809-b0798e1608f0