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

drivers/tty/serial/amba-pl011.c

@@ -1236,10 +1236,6 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
 
 #else
 /* Blank functions if the DMA engine is not available */
-static inline void pl011_dma_probe(struct uart_amba_port *uap)
-{
-}
-
 static inline void pl011_dma_remove(struct uart_amba_port *uap)
 {
 }