media: airspy: fix memory leak in airspy probe

The commit ca9dc8d0 ("media: airspy: respect the DMA coherency
 rules") moves variable buf from stack to heap, however, it only frees
buf in the error handling code, missing deallocation in the success
path.

Fix this by freeing buf in the success path since this variable does not
have any references in other code.

Fixes: ca9dc8d0 ("media: airspy: respect the DMA coherency rules")
Reported-by:  <syzbot+bb25f85e5aa482864dc0@syzkaller.appspotmail.com>
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Reviewed-by: Tommaso Merciai <tommaso.merciai@amarulasolution.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>