cxgbe(4): Do not read hardware registers to determine the number of ports.

PORTVEC obtained from the firmware is the authoritative source of this
information, and nports (calculated from PORTVEC) is available by the
time t4_port_init runs.

Sponsored by: Chelsio Communications

(cherry picked from commit 4d1362cdc7375984a48f5f0048b1fe909524d21d)