metal_sleep_usec() used portTICK_PERIOD_MS to determine how long to
sleep. If the FreeRTOS tick rate is set to greater than 1000, this will
be 0, resulting in a divide by zero error at run time. This changes it
to use pdMS_TO_TICKS(), which can accomodate higher tick rates.
Signed-off-by: Ed Mooring <ed.mooring@linaro.org>