bnx2x: fix slowpath null crash

When "NETDEV WATCHDOG: em4 (bnx2x): transmit queue 2 timed out" occurs,
BNX2X_SP_RTNL_TX_TIMEOUT is set. In the function bnx2x_sp_rtnl_task,
bnx2x_nic_unload and bnx2x_nic_load are executed to shutdown and open
NIC. In the function bnx2x_nic_load, bnx2x_alloc_mem allocates dma
failure. The message "bnx2x: [bnx2x_alloc_mem:8399(em4)]Can't
allocate memory" pops out. The variable slowpath is set to NULL.
When shutdown the NIC, the function bnx2x_nic_unload is called. In
the function bnx2x_nic_unload, the following functions are executed.
bnx2x_chip_cleanup
    bnx2x_set_storm_rx_mode
        bnx2x_set_q_rx_mode
            bnx2x_set_q_rx_mode
                bnx2x_config_rx_mode
                    bnx2x_set_rx_mode_e2
In the function bnx2x_set_rx_mode_e2, the variable slowpath is operated.
Then the crash occurs.
To fix this crash, the variable slowpath is checked. And in the function
bnx2x_sp_rtnl_task, after dma memory allocation fails, another shutdown
and open NIC is executed.

CC: Joe Jin <joe.jin@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Acked-by: Ariel Elior <aelior@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>