snd-firewire-lib: fix WARN_ON in amdtp_stream_set_rate() at 48.0kHz

The cause is my typo in amdtp_stream_params table.
snd-firewire-lib: replace fix value with macro exists

In receive_stream_callback(), to calculate the number of packet, fix value "4"
is used. This "4" is the number of bytes in header of isochronous packet and
the same value as RECEIVE_PACKET_HEADER_SIZE.

This commit replace the fix value with the macro.