ARC: cacheflush: No need to retain DC_CTRL from __before_dc_op()

That is because __after_dc_op() already reads it for status check, so it
is better anyways to use that "newer" value.

Also reduces the clutter in callers for passing from/to these routines.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>