seq: Fix wrong FB direction at snd_seq_create_ump_block()

seq: Fix wrong FB direction at snd_seq_create_ump_block()

snd_seq_create_ump_block() receives a snd_ump_block_info_t data at the
creation of a FB and updates its associated sequencer port, but it
handled the port direction incorrectly.  The UMP / port direction
means the connectivity, but the current code translated other way
round.  The correct translation should be that input = receiver, i.e.
a writable port for applications, and output = source, a readable port
for applications.

This patch corrects the translation, and add more comments to the
direction definition.

Signed-off-by: Takashi Iwai <tiwai@suse.de>