cxgbe(4): meminfo should get the TLS region's limits from the hardware.

meminfo is meant for debugging and it should read hardware configuration
directly when possible instead of relying on values queried from the
firmware. Rename the region to "TLSKey region" to match other drivers
while here.

Sponsored by: Chelsio Communications

(cherry picked from commit f88b31885c4fd1f80bed2c3b3f2eb05f98367f31)