[hermon] Add missing __attribute__ (( packed ))

[hermon] Add missing __attribute__ (( packed ))

On 64-bit builds, MLX_DECLARE_STRUCT() produces a structure that is
always a multiple of 64 bits long, causing the HCR structure to be
over-length by one dword.  This in turn causes hermon_cmd() to write
beyond the end of the HCR, which causes commands to fail.

Reported-by: Itay Gazit <itayg@mellanox.co.il>
Signed-off-by: Michael Brown <mcb30@ipxe.org>