[ALSA] mpu-401: do not require an ACK byte for the ENTER_UART command

The original MPU-401 does not send an ACK byte after processing the
ENTER_UART command, so we better do not try to wait for it.
Many clones get this wrong and do send an ACK, but when interpreting it
as MIDI, it is just a harmless Active Sensing message, and it is likely
to be read and discarded by the interrupt handler before the MIDI input
device is triggered.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>