ena: Use device_set_desc in probe

During probe the driver created a temporary buffer to which the value of
DEVICE_DESC constant was printed. This buffer was then copied to the
device structure using device_set_desc_copy. Since the value of this
string is exactly the same for every device using the ENA driver, using
sprintf is unnecessary, and device_set_desc can be used instead.

Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.