[serial] Enable UART FIFOs

Escape sequences received via the serial console can fail since the
cpu_nap() in getchar_timeout() can delay processing for more than the
time it takes for a single character to arrive.

Fix by enabling the UART FIFOs.

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/serial.c

 	/* disable interrupts */
 	uart_writeb(0x0, UART_BASE + UART_IER);
 
-	/* disable fifo's */
-	uart_writeb(0x00, UART_BASE + UART_FCR);
+	/* enable fifos */
+	uart_writeb(0x01, UART_BASE + UART_FCR);
 
 	/* Set clear to send, so flow control works... */
 	uart_writeb((1<<1), UART_BASE + UART_MCR);