bnxt_en: Fix statistics context reservation logic for RDMA driver.

The current logic assumes that the RDMA driver uses one statistics
context adjacent to the ones used by the network driver.  This
assumption is not true and the statistics context used by the
RDMA driver is tied to its MSIX base vector.  This wrong assumption
can cause RDMA driver failure after changing ethtool rings on the
network side.  Fix the statistics reservation logic accordingly.

Fixes: 780baad4 ("bnxt_en: Reserve 1 stat_ctx for RDMA driver.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>