serial: amba-pl011: fix build regression

When CONFIG_DMA_ENGINE is disabled, the driver now fails to build:

drivers/tty/serial/amba-pl011.c: In function 'pl011_unthrottle_rx':
drivers/tty/serial/amba-pl011.c:1822:16: error: 'struct uart_amba_port' has no member named 'using_rx_dma'
 1822 |         if (uap->using_rx_dma) {
      |                ^~
drivers/tty/serial/amba-pl011.c:1823:20: error: 'struct uart_amba_port' has no member named 'dmacr'
 1823 |                 uap->dmacr |= UART011_RXDMAE;
      |                    ^~
drivers/tty/serial/amba-pl011.c:1824:32: error: 'struct uart_amba_port' has no member named 'dmacr'
 1824 |                 pl011_write(uap->dmacr, uap, REG_DMACR);
      |                                ^~

Add the missing #ifdef check around these field accesses, matching
what other parts of this driver do.

Fixes: 2bcacc1 ("serial: amba-pl011: Fix RX stall when DMA is used")
Cc: stable <stable@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411140617.nkjeHhsK-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20241115110021.744332-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: arndb

drivers/tty/serial/amba-pl011.c

@@ -1819,10 +1819,12 @@ static void pl011_unthrottle_rx(struct uart_port *port)
 
 	pl011_write(uap->im, uap, REG_IMSC);
 
+#ifdef CONFIG_DMA_ENGINE
 	if (uap->using_rx_dma) {
 		uap->dmacr |= UART011_RXDMAE;
 		pl011_write(uap->dmacr, uap, REG_DMACR);
 	}
+#endif
 
 	uart_port_unlock_irqrestore(&uap->port, flags);
 }