The error report timer is non-negative; make it FT_UINT16.

This squelches an assertion.

Use %u to print it, while we're at it.

Reformat the generated named fields to match the others.

Use proto_tree_add_item for the beacon type.

Change-Id: I1dcf161141638de8d7314a3733391a63e0608f67
Reviewed-on: https://code.wireshark.org/review/3711
Reviewed-by: Guy Harris <guy@alum.mit.edu>