ipmi: kcs_bmc_aspeed: Fix less than zero comparison of a unsigned int

The comparisons of the unsigned int hw_type to less than zero always
false because it is unsigned. Fix this by using an int for the
assignment and less than zero check.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 9d2df9a0ad80 ("ipmi: kcs_bmc_aspeed: Implement KCS SerIRQ configuration")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20210616162913.15259-1-colin.king@canonical.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>