ALSA: usb-audio: Relax __free() variable declarations

ALSA: usb-audio: Relax __free() variable declarations

We used to have a variable declaration with __free() initialized with
NULL.  This was to keep the old coding style rule, but recently it's
relaxed and rather recommends to follow the new rule to declare in
place of use for __free() -- which avoids potential deadlocks or UAFs
with nested cleanups.

Although the current code has no bug, per se, let's follow the new
standard and move the declaration to the place of assignment (or
directly assign the allocated result) instead of NULL initializations.

Note that there are still a few remaining __free(kfree) with NULL
initializations; they are because of the code complexity (the data
size calculation).

Fixes: 43d4940c944c ("ALSA: usb: scarlett2: Clean ups with guard() and __free()")
Fixes: 46757a3e7d50 ("ALSA: FCP: Add Focusrite Control Protocol driver")
Fixes: f7d306b47a24 ("ALSA: usb-audio: Fix a DMA to stack memory bug")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20251216140634.171890-12-tiwai@suse.de