ieee1394: nodemgr: check info_length in ROM header earlier

The whole ROM area which is covered by the crc_length field of the ROM
header was fetched before the info_length field was checked for correct
general ROM format.  This might be wasteful or even dangerous with nodes
with minimal ROM, nonstandard ROM, or corrupt ROM.

Perform this check at the earliest opportunity.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>