usb: isp1760: Replace mdelay with msleep in isp1760_stop

isp1760_stop() is never called in atomic context.

The call chain ending up at isp1760_stop() is:
[1] isp1760_stop() <- isp1760_shutdown()

isp1760_shutdown() is set as ".shutdown" in struct hc_driver.
isp1760_stop() is also set as ".stop" in hc_driver.
These functions are not called in atomic context.

Despite never getting called from atomic context, isp1760_stop()
calls mdelay() to busily wait.
This is not necessary and can be replaced with msleep() to
avoid busy waiting.

This is found by a static analysis tool named DCNS written by myself.
And I also manually check it

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>