mtd: rawnand: diskonchip: Replace mdelay with usleep_range in doc_probe

doc_probe() is never called in atomic context.

doc_probe() is only called by init_nanddoc(), which is only set as
a parameter of module_init().
This function is not called in atomic context.

Despite never getting called from atomic context, doc_probe()
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: Boris Brezillon <boris.brezillon@bootlin.com>