mailbox: Fix a couple of trivial static checker issues

This patch deals with a few spelling, white space and type
warnings reported by Intel's Kbuild Test Robot.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>