mmc: sh_mobile_sdhi: improve prerequisite for hw_reset

We need a SCC unit for hw_reset. Those units can only be described in
of_data. So, of_data and a valid SCC offset are prerequisites for
enabling the hw_reset capability. Merge the two 'if' conditions into one
and add a check for an scc offset.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>