mmc: bcm2835_sdhci: Speed up mmc writes.

The linux kernel driver for this module does not use a delay when
writing to the SDHCI_BUFFER register. This patch mimics that behavior
in order to speed up the mmc writes on the Raspberry Pi.

Signed-off-by: Alex Deymo <deymo@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>