ARM: OMAP: Remove unwanted type casts and fix the compiler warning.

This patch fixes the compiler warning "assignment from incompatible
pointer type"  in dmtimer.c and removes the tye casts. These warnings
were suppressed by type catsing.

The proposed fix was suggested by Russell King <rmk+kernel@arm.linux.org.uk>

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>