drivers: ipmi: fix off-by-one bounds check that leads to a out-of-bounds write

The end of buffer check is off-by-one since the check is against
an index that is pre-incremented before a store to buf[]. Fix this
adjusting the bounds check appropriately.

Addresses-Coverity: ("Out-of-bounds write")
Fixes: 51bd6f29 ("Add support for IPMB driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20200114144031.358003-1-colin.king@canonical.com>
Reviewed-by: Asmaa Mnebhi <asmaa@mellanox.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>