ALSA: usb-audio: Convert the last strlcpy() usage

The last remaining usage of strlcpy() in USB-audio driver is the setup
of the card longname string.  Basically we need to know whether any
non-empty string is set or not, and no real length is needed.
Refactor the code and use strscpy() instead.  After this change,
strlcpy() is gone from all sound/* code.

Link: https://lore.kernel.org/r/20210115100437.20906-1-tiwai@suse.de


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