iwl3945: update iwl3945_apm_init()

Update iwl3945_apm_init() to set up device registers in sequence most recently
recommended by factory.

Add resets for APMG interrupts and radio chip, formerly done only in
iwl3945_apm_reset(); moving them here assures that apm_init() will do
a complete job of preparing hardware not only after platform boot,
but also after apm_stop() has executed (due to rfkill, ifconfig down,
driver unload, etc.).  This is in preparation to completely remove apm_reset().

Add some comments.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>