ide: don't try to unregister interfaces if 'initializing' in ide_register_hw()

Host drivers using ide_register_hw() and 'initializing == 1':

* ide-pnp
  - depends on ISA

* ide_arm 
  - ARM arch specific
  - initialized before all other host drivers

* ide-cris
  - CRIS arch specific => IDE_ARCH_OBSOLETE_INIT is not defined
  - broken

* ide-h8300
  - H8300 arch specific => IDE_ARCH_OBSOLETE_INIT is not defined, no PCI

* buddha/q40/gayle/macide/falconide
  - M68K arch specific => IDE_ARCH_OBSOLETE_INIT is not defined, no PCI

Since the only host drivers which probe interfaces before the above ones are:

* ali14xx/dtc2278/ht6560b/qd65xx/umc8672
  - depend on ISA
  - require IDE_ARCH_OBSOLETE_INIT=y to work

* PCI ones
  - depend on PCI

don't try to unregister interfaces if 'initializing == 1' in ide_register_hw()
(it is possible that built-in host drivers will claim all IDE interfaces but
later ide-pnp host driver will try to unregister them - this change fixes it).

Also skip hwif->hold check if 'initializing == 1' since it is set only by:

* pmac
  - PPC && PMAC specific => no ISA

* au1xxx-ide
  - MIPS && SOC_AU1200 specific => no ISA

and use ide_find_port() helper to find free ide_hwifs[] slot.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>