At some point after stable/7 the ACPI and ISA interfaces to the IPMI controller

At some point after stable/7 the ACPI and ISA interfaces to the IPMI controller
no longer have the parent in the device tree.  This causes the identify
function in ipmi_isa.c to attempt to probe and poke at the ISA IPMI interface

Move the check for ipmi_attached out of the ipmi_isa_attach function and into
the ipmi_isa_identify function.  Remove the check of the device tree for
ipmi devices attached.

This probing appears to make Broadcom management firmware on Dell machines
crash and emit NMI EISA warnings at various times requiring power cycles
of the machines to restore.

Bump MAX_TIMEOUT to 6 seconds as a hack for super slow IPMI interfaces that
need longer to respond to our intial probes on startup.

Tested on Dell R410, R510, R815, HP DL160G6

This is MFC candidate for 9.2R

Reviewed by:	peter
MFC after:	2 weeks
Sponsored by:	Yahoo! Inc.