cxgb3: remove VLA usage

Remove VLA usage and change the 'len' argument to a u8 and use a 256
byte buffer on the stack. Notice that these lengths are limited by the
encoding field in the VPD structure, which is a u8 [1].

[1] https://marc.info/?l=linux-netdev&m=152044354814024&w=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>