Use usleep_range instead of msleep when waiting for command
completion. Most SMBus commands complete in less than 2 jiffies so
this brings a pleasant performance boost.

Strongly inspired from a similar change by Olivier Sobrie to the
i2c-isch driver.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Olivier Sobrie <olivier@sobrie.be>