bnxt_en: don't disable an already disabled PCI device

If device is already disabled in reset path and PCI io error is
detected before the device could be enabled, driver could
call pci_disable_device() for already disabled device. Fix this
problem by calling pci_disable_device() only if the device is already
enabled.

Fixes: 6316ea6d

 ("bnxt_en: Enable AER support.")
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>