omap: McBSP: Make the free variable update more readable

Using true/false instead of 1/0 to update the free variable.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>