cxgb4: Fixed the code to use correct length for part number

Previously it was using the length value of serial number.
Also added macro for VPD unique identifier (0x82).

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>