ide-generic: minor fix for mips

Move ide_probe_legacy() call to ide_generic_init() so it fails
early if necessary and returns the proper error value (nowadays
ide_default_io_base() is used only by ide-generic).

Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>