simplify the while-loop in dissect_pmip6_opt_cr()

Bug: 10676
Change-Id: I5614c640d3fd23196984eb2bb013ba15fb318305
Reviewed-on: https://code.wireshark.org/review/5200
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>