mailbox/omap: use variable name for sizeof() operator

Fix the code formatting to use the kernel preferred style
of using the actual variables to determize the size using
the sizeof() operator. This fixes the corresponding checkpatch
warning as well.

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