mpr: fix copying of event_mask

Before the commit 6cc44223cb6717795afdac4348bbe7e2a968a07d the
field event_mask was fully copied to the EventMasks field.
After this commit the event_mask (uint8_t) is 4 times casted to
EventMask (uint32_t). Because of that 24 bits of each event_mask array
is lost.

This commits brings back simple copying of field, and after words
converting 32 bits field to the requested endian.

I don't think we need more sophisticated method,
as the array is of size 4 (for 32 bits version).

Reviewed by: imp
MFC after: 1 week
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D39562