Declare packed struct ata_params as 2-byte-aligned

This avoids gcc9 warning about unaligned access to the structure when
casting to uint16_t pointer type.

Submitted by: imp
Reviewed by: imp
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22888

(cherry picked from commit 36947e1f4deacec2a88bba226e9b46efebe8655e)