ALSA: usb-audio: rotate standard MIDI output port scan

ALSA: usb-audio: rotate standard MIDI output port scan

snd_usbmidi_standard_output() iterates output ports in ascending order
and drains each active port until the URB is full. On interfaces where
multiple USB-MIDI cables share one endpoint, sustained traffic on a
lower-numbered port can consume every refill before higher-numbered
ports are even examined.

That behavior dates back to the original implementation and still
applies with the current multi-URB output path. snd_usbmidi_do_output()
can refill several idle URBs in one pass, but each refill restarts the
scan at port 0, so a busy lower-numbered port can keep higher-numbered
ports from making progress at all.

Use ep->current_port as the starting point of the scan and advance it
after each URB fill. This keeps the existing packet formatting and
per-port state handling intact while preventing persistent starvation of
higher-numbered ports.

Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260323-usbmidi-port-fairness-v1-1-2d68e97592a1@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>