IB/ipath: Limit length checksummed in eeprom

The small eeprom that holds the GUID etc. contains a data-length, but if 
the actual eeprom is new or has been erased, that byte will be 0xFF,
which is greater than the maximum physical length of the eeprom, and
more importantly greater than the length of the buffer we vmalloc'd.
Sanity-check the length to avoid the possbility of reading past end of
buffer.

Signed-off-by: Michael Albaugh <Michael.Albaugh@Qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>