common: bouncebuf: Add missing cast to dma_addr_t

Fix the following warning produced on qemu-x86_64_defconfig:

"
common/bouncebuf.c: In function ‘bounce_buffer_stop’:
common/bouncebuf.c:82:34: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   82 |                 dma_unmap_single((dma_addr_t)state->bounce_buffer,
      |                                  ^
"

The warning is valid, the pointer has to be up-cast first.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Andrew Davis <afd@ti.com>