bnxt_en: Unhandled async event type 76

The completion event type 76 is not supported by the driver. Instead of
flooding the dmesg with "Unknown event type" messages when this event
occurs, move the print under debug level.

MFC-After: 3 days
Differential-Revision: https://reviews.freebsd.org/D49731
(cherry picked from commit 6450d937955fcd1ab9034c49d53306e882c4a281)