qtnfmac: Tidy up DMA mask setting

As the only caller of dma_supported() outside of DMA API internals, the
qtfnmac driver stands out and invites scrutiny. Thankfully, it's not
being used for evil, but it is entirely redundant, since it open-codes a
check that the DMA mask setting functions are going to perform anyway.
In fact, the whole qtnf_pcie_init_dma_mask() function is nothing more
than a rather long-winded implementation of dma_set_mask_and_coherent(),
so let's just use that directly.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>