sfxge(4): cope with clang warning on negative shift

clang 4.0.1-6 on Ubuntu generates false positive warning that shift
is negative. It is done regardless of the fact that the branch is
not taken because of previous check.

The warning is generate in EFX_INSERT_NATIVE32 used by
EFX_INSERT_FIELD_NATIVE32. All similar cases are fixed as well.

It is undesirable to suppress the warning completely.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18229