mmc: sdhci-sirf: fix 8bit width enable by overwriting set_bus_width

the implementation of CSR SDHCI controller is a modified version of
the one described in the 1.0 specification, and not a normal 3.0
controller.
and 8bit-width enable bit of CSR MMC hosts is 3, while stardard hosts
use bit 5.
this patch fixes the functionality of 8bit transfer in mmc controllers
and improve performance for mmc0 a lot.

Signed-off-by: Minda Chen <Minda.Chen@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Reviewed-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>