pcan_add_channels() is never called in atomic context.

pcan_add_channels() is only called by pcan_probe(), which is only set as
".probe" in struct pcmcia_driver.

Despite never getting called from atomic context, pcan_add_channels()
calls mdelay() to busily wait.
This is not necessary and can be replaced with usleep_range() 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: Marc Kleine-Budde <mkl@pengutronix.de>