spi: loopback-test: add elapsed time check

This adds checks whether the elapsed time is longer than the minimam
estimated time.  The estimated time is calculated with the total
transfer length per clock rate and optional spi_transfer.delay_usecs.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>