staging: regulator: hi6421v600-regulator: don't use usleep_range for off_on_delay

The regulator's core already handles it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4f17adca2b04c401e5de5556700638648ce25c0c.1597647359.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>