driver core: Use the ktime_us_delta() helper

Use the ktime_us_delta() helper to measure the driver probe time. Given the
helpers already returns an s64 value, let's drop the unnecessary casting to
s64 as well. There is no functional change.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20200803033343.1178-1-yuzenghui@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>