samples: olimex_stm32_e407: fix coverity warning

Fix a Coverity warning when using %d to print the result
of a sizeof(...)

Since the result of the sizeof() use in the example
will always fit in a int, simply cast the result
to a int so the %d is always correct.

Fixes: #18373

Signed-off-by: Erwin Rol <erwin@erwinrol.com>