sh: add init member to pci_channel data

This patch adds an init callback to struct pci_channel and makes sure
it is initialized properly. Code is added to call this init function
from pcibios_init(). Return values are adjusted and a warning is is
printed if init fails.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>