ipmi: ipmb: Fix off-by-one size check on rcvlen

There is an off-by-one bounds check on the rcvlen causing a potential
out of bounds write on iidev->rcvmsg. Fix this by using the >= operator
on the bounds check rather than the > operator.

Addresses-Coverity: ("Out-of-bounds write")
Fixes: 0ba0c3c5d1c1 ("ipmi:ipmb: Add initial support for IPMI over IPMB")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20211005151611.305383-1-colin.king@canonical.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>