omap: Use resource_size

Use the resource_size function instead of manually calculating the
resource size. This reduces the chance of introducing off-by-one errors
and actually fixes one in mailbox.c.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>
diff --git a/arch/arm/mach-omap2/gpmc.c b/arch/arm/mach-omap2/gpmc.c
index 1587682..f865756 100644
--- a/arch/arm/mach-omap2/gpmc.c
+++ b/arch/arm/mach-omap2/gpmc.c
@@ -366,7 +366,7 @@
 	if (r < 0)
 		goto out;
 
-	gpmc_cs_enable_mem(cs, res->start, res->end - res->start + 1);
+	gpmc_cs_enable_mem(cs, res->start, resource_size(res));
 	*base = res->start;
 	gpmc_cs_set_reserved(cs, 1);
 out: