net: atlantic: simplify hw_get_fw_version() usage

hw_get_fw_version() never fails, so this patch simplifies its
usage by utilizing return value instead of output argument.

Signed-off-by: Nikita Danilov <ndanilov@marvell.com>
Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>