bnxt_en: Improve AER slot reset.

commit bae361c5 upstream.

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>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>