zephyr: Wrap sleep value with K_MSEC

Zephyr timeout API is changing and will use opaque value (k_timeout_t)
instead of raw values. K_MSEC is used to convert raw milliseconds value
to k_timeout_t.

This change is backward compatible so can be merged independently.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>