omap: McBSP: Fix potential memory leak in omap_mcbsp_remove

Function omap_mcbsp_probe allocates struct omap_mcbsp *mcbsp but it is not
freed in omap_mcbsp_remove. Fix this, remove unneeded structure cleanups
and clk_disable calls since they are not needed here.

This is not problem currently but becomes if the mcbsp driver is ever
modularized.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>