[ISDN] HiSax netjet_s: code movement, prep for hotplug

1) Remove CONFIG_PCI ifdefs.  PCI is required in Kconfig.

2) Break up setup_netjet_s() into three separate internal functions.
This helps facilitate upcoming use of PCI hotplug API, and in addition
makes the code much easier to follow.

No code is changed, just moved around.  I even kept the out-of-favor
"return(0)" style used in the current source code.

Signed-off-by: Jeff Garzik <jeff@garzik.org>