firmware_handler: minor code cleanup
firmware_handler: minor code cleanup

Just return based on the result of the condition instead of deliberately
checking each condition.

Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>
Change-Id: I291d8c388d707fe880e70374c3746e595d410632

openbmc/phosphor-ipmi-flash