[realtek] Allow reaction time between writing RTL_CAPR and reading RTL_CR

Some older RTL8139 chips seem to not immediately update the
RTL_CR.BUFE bit in response to a write to RTL_CAPR.  This results in
iPXE seeing a spurious zero-length received packet, and thereafter
being out of sync with the hardware's RX ring offset.

Fix by inserting an extra PCI read cycle after writing to RTL_CAPR, to
give the chip time to react before we next read RTL_CR.

Reported-by: Gelip <mrgelip@gmail.com>
Tested-by: Gelip <mrgelip@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>