mmc: sdhci-xenon: remove pointless struct xenon_priv *priv

It was never used and leave a long standing compile warning:

drivers/mmc/host/sdhci-xenon.c: In function 'xenon_probe':
drivers/mmc/host/sdhci-xenon.c:447:21: warning: variable 'priv' set but
not used [-Wunused-but-set-variable]

Remove it to fix the warning.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>