net/bnxt: increase max burst size for vector path

net/bnxt: increase max burst size for vector path

Increase the maximum supported burst size for the bnxt vector
mode PMD from 32 to 64.

With larger burst sizes, per-burst overhead is amortized over more
packets, improving overall performance. For small packets this has
been measured to provide a 4-10% increase in single-core throughput
with testpmd iofwd.

Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Lance Richardson <lance.richardson@broadcom.com>