cmd640: add ->init_dev method

Convert the driver to use ->init_dev method instead of open-coding devices
init in cmd640x_init().

While at it:

- fix printk()-s to use KERN_INFO level instead of the default KERN_ERR

- use DRV_NAME define in printk()-s

- set proper ->pio_mask also for CONFIG_BLK_DEV_CMD640_ENHANCED=n

There should be no functional changes caused by this patch
(except fixing printk()-s levels).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>