MFD: ucb1x00-core: disable mcp clock when bus is not required

The ucb1x00-core was leaving the mcp clock enabled indefinitely after
probe.  This needlessly wastes power.  Add the necessary disables to
ensure that the clock remains off when we don't need it.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>