mailbox/omap: add blank lines after declarations

Fix couple of checkpatch warnings of the type,
"WARNING: Missing a blank line after declarations"

Also, fixed a warning about a space after a typecast
while at this.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>