arch/arm/mach-omap2/dma.c: Convert IS_ERR result to PTR_ERR

This code elsewhere returns a negative constant to an indicate an error,
while IS_ERR returns the result of a >= operation.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression x;
@@

 if (...) { ...
- return IS_ERR(x);
+ return PTR_ERR(x);
}
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>