[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>