spi: core: Replace msleep with usleep_range to get more accurate sleep time

Fixes below checkpatch warning:
WARNING: msleep < 20ms can sleep for up to 20ms; see Documentation/timers/timers-howto.txt
+               msleep(10);

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>