mmc: sdhci-spear: fix platform_data usage

sdhci-spear is unsafe should a probe fail or defer, since it overwrites
the platform_data with its own driver-private data.  It's trivial to
fix as SDHCI allows for driver-private data to be appended to its own
structure - we just need to arrange the code to allow this.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Chris Ball <chris@printf.net>