dmaengine: sf-pdma: pdma_desc memory leak fix

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

dmaengine: sf-pdma: pdma_desc memory leak fix

Commit b2cc5c465c2c ("dmaengine: sf-pdma: Add multithread support for a
DMA channel") changed sf_pdma_prep_dma_memcpy() to unconditionally
allocate a new sf_pdma_desc each time it is called.

The driver previously recycled descs, by checking the in_use flag, only
allocating additional descs if the existing one was in use. This logic
was removed in commit b2cc5c465c2c ("dmaengine: sf-pdma: Add multithread
support for a DMA channel"), but sf_pdma_free_desc() was not changed to
handle the new behaviour.

As a result, each time sf_pdma_prep_dma_memcpy() is called, the previous
descriptor is leaked, over time leading to memory starvation:

unreferenced object 0xffffffe008447300 (size 192):
comm "irq/39-mchp_dsc", pid 343, jiffies 4294906910 (age 981.200s)
hex dump (first 32 bytes):
00 00 00 ff 00 00 00 00 b8 c1 00 00 00 00 00 00 ................
00 00 70 08 10 00 00 00 00 00 00 c0 00 00 00 00 ..p.............
backtrace:
[<00000000064a04f4>] kmemleak_alloc+0x1e/0x28
[<00000000018927a7>] kmem_cache_alloc+0x11e/0x178
[<000000002aea8d16>] sf_pdma_prep_dma_memcpy+0x40/0x112

Add the missing kfree() to sf_pdma_free_desc(), and remove the redundant
in_use flag.