mmc: sdhci-sirf: fix the tuning count in platform_execute_tuning

hardware has 16bit to record the tuning count, so fix it to 16384.
at the same time, tuned_phases[SIRF_TUNING_COUNT] is useless as the
array is never used, so move it to a variant.

Signed-off-by: Weijun Yang <Weijun.Yang@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>