Remove atapi_error_t.

While at it:
* replace 'HWIF(drive)' by 'drive->hwif'

v2:
* Add {ILI,EOM,LFS}_ERR defines to <linux/hdreg.h>.
Acked-by: NSergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: NBartlomiej Zolnierkiewicz <bzolnier@gmail.com>