cxgbe(4): Make no assumptions about the start and size of the L2T table.

Do not assume that the table starts at index 0 and is typically 4K in
size. The only thing the driver needs to verify is that its use of
F_SYNC_WR doesn't collide with the L2T hwidx range.

Reviewed by: jhb
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D46028

(cherry picked from commit cfcfd3c7bf5b60da42b13ac5d8085c762613c302)