sfxge(4): use correct port number in sensor decoding

The port mask used for per-port sensors in mcdi_sensor_map
assumes zero-based port numbering. The port mask used in
the code is based on the one-based MCDI port number.

Fix this to lookup the correct per-port sensors, and to
allow reporting of sensor events from higher port numbers.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8947