x1000: revamp MSC driver card detection logic
Debounce logic now handles both removal and insertion and verifies
the detection is stable for 100ms before taking any action.
This solves the bootloader "file not found" issue on the Shanling Q1.
It seems a false removal event was generated, causing the filesystem
to be automatically unmounted. Probably this is due to some transient
noise at boot. Delays didn't solve the problem, probably because the
bogus hotplug event was queued, and normal mdelay() would simply delay
event delivery.
Change-Id: I8b03fb3550309f5a7ab4be0be7465a3dab2d3450