Power off the SD card in `DISK_IOCTL_CTRL_DEINIT`, instead of only
waiting for the card to be idle then doing nothing. This is a safer
state for `DISK_IOCTL_CTRL_DEINIT`, which is documented as preparing the
disk to be removed from the system. It also has the advantage of
lowering power consumption while de-initialized.
Signed-off-by: Jordan Yates <jordan@embeint.com>
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