drivers/storage: Remove set_control_reg

Remove unused field in generic SD/MMC controller data structure.

TEST=Build and run on Galileo Gen2

Change-Id: I7169dca07509a6f2513d62b593742daf764010b2
Signed-off-by: Lee Leahy <Leroy.P.Leahy@intel.com>
Reviewed-on: https://review.coreboot.org/19629
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
2 files changed