libopenbmc_intf: fix various warnings

Trying to compile with stricter warnings results in numerous
errors, such as ignored results and unsigned/signed comparisons.
Fix these in libopenbmc_intf.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia0b34fdc27b32675082741a0f3c18f0b538be108
4 files changed