bnxt_en: Fix NULL pointer dereference at bnxt_free_irq().

When open fails during ethtool -L ring change, for example, the driver
may crash at bnxt_free_irq() because bp->bnapi is NULL.

If we fail to allocate all the new rings, bnxt_open_nic() will free
all the memory including bp->bnapi.  Subsequent call to bnxt_close_nic()
will try to dereference bp->bnapi in bnxt_free_irq().

Fix it by checking for !bp->bnapi in bnxt_free_irq().

Fixes: e5811b8c

 ("bnxt_en: Add IRQ remapping logic.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>