bnxt_en: Improve AER slot reset.

Improve the slot reset sequence by disabling the device to prevent bad
DMAs if slot reset fails.  Return the proper result instead of always
PCI_ERS_RESULT_RECOVERED to the caller.

Fixes: 6316ea6d ("bnxt_en: Enable AER support.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>