ide: legacy PCI bus order probing fixes

IDE PCI host drivers should register themselves with IDE core only when
IDE driver is built-in, otherwise (IDE driver is modular and thus IDE PCI
host drivers are also modular) the code has no effect and just complicates
the probing.

Fix it by adding new config option CONFIG_IDEPCI_PCIBUS (defined only when
needed and invisible to the user) and covering by #ifdef/#endif the code
in question.  It turned out that "ide=reverse" was silently accepted but did
nothing in case when IDE driver was modular, this is fixed now.

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