media: airspy: fix memory leak in airspy probe

In the Linux kernel, the following vulnerability has been resolved:

media: airspy: fix memory leak in airspy probe

The commit ca9dc8d06ab6 ("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.