tty/amba-pl011: fix a -Wunused-function warning

pl011_dma_probe() is only used in pl011_dma_startup() which does only
exist when CONFIG_DMA_ENGINE=y, so remove the unused dummy version to
silence the warning.

Signed-off-by: Qian Cai <cai@lca.pw>
Link: https://lore.kernel.org/r/1568726340-4518-1-git-send-email-cai@lca.pw


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>