powerpc: mpc85xx: Fix check for CONFIG_SDCARD

Commit d433c74e ("Convert CONFIG_SDCARD et al to Kconfig") converted
SYS_EXTRA_OPTIONS=SDCARD or SPIFLASH to config options CONFIG_SDCARD and
CONFIG_SPIFLASH, but left one occurance unchanged.

Fix this.

Fixes: d433c74e ("Convert CONFIG_SDCARD et al to Kconfig")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Pali Rohár <pali@kernel.org>