usb: chipidea: don't redefine __ffs()

chipidea's ffs_nr() is pretty much what __ffs() does.

Use that one instead.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[rebased on top of debug infrastructure rework]
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>