alpha: Use str_yes_no() helper in pci_dac_dma_supported()

Remove hard-coded strings by using the str_yes_no() helper function.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Matt Turner <mattst88@gmail.com>