net/ixgbe/base: fix firmware command checksum error

net/ixgbe/base: fix firmware command checksum error

When software sends commands to firmware using the host
slave command interface, firmware fails to receive the
command due to a checksum failed error, as the checksum is
not being correctly set by the driver software.

This patch sets command checksum to the default value of
0xFF, as per the datasheet, therefore the checksum won't
be checked by firmware.

Fixes: 86b8fb293fdf ("ixgbe/base: add sw-firmware sync for resource sharing on X550em_a")
Fixes: 0790adeb5675 ("ixgbe/base: support X550em_a device")

Signed-off-by: Beilei Xing <beilei.xing@intel.com>