bnxt_en: Don't proceed in .ndo_set_rx_mode() when device is not in open state.

Check the BNXT_STATE_OPEN flag instead of netif_running() in
bnxt_set_rx_mode().  If the driver is going through any reset, such
as firmware reset or even TX timeout, it may not be ready to set the RX
mode and may crash.  The new rx mode settings will be picked up when
the device is opened again later.

Fixes: 230d1f0d ("bnxt_en: Handle firmware reset.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>