bluetooth: Fix usage of MTU, buffer sizes and return values of encode/decode methods

bluetooth: Fix usage of MTU, buffer sizes and return values of encode/decode methods

Add explanation why minimal bitpool value is used in SBC codec as initial
bitpool value for A2DP source.

Set buffer size for reading/writing from/to A2DP socket to exact link MTU
value. This would ensure that A2DP codec does not produce larger packet as
maximal possible size which can be sent.

Because A2DP socket is of SOCK_SEQPACKET type, it is guaranteed that
we do not read two packets via one read/recvmsg call.

Properly check for all return values of encode/encode methods of A2DP codec
functions. They may fail at different levels. Also encode or decode API
method may return zero length buffer (e.g. because of algorithmic delay of
codec), so do not fail in this case.