qa/src/xval.c: safer bit-field assignment
qa/src/xval.c: safer bit-field assignment

Assigning -1 (PM_TYPE_NOSUPPORT) to a 8-bit field sort of demands
truncation, so make it explicit rather than implicit.
Found by the FreeBSD 10.2 C compiler.

adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp