net: txgbe: Fix unsigned comparison to zero in txgbe_calc_eeprom_checksum()

The error checks on checksum for a negative error return always fails because
it is unsigned and can never be negative.

Fixes: 049fe536

 ("net: txgbe: Add operations to interact with firmware")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>