iwlwifi: pcie: avoid unnecessary work if NIC is disconnected

When the NIC is disconnected from PCI bus, we are not
able to access it anymore. Check the status to avoid
some unnecessary work so can improve the performance.
It will help to make PCI bus rescan to bring back the
device much faster.

The real test is able to improve 7 seconds.

[w/o patch] It takes around 9 seconds
..
2018-04-20T01:22:39.691929-07:00 WARNING kernel:
[   66.335881] Timeout waiting for hardware access (CSR_GP_CNTRL 0xffffffff)
..
2018-04-20T01:22:48.101094-07:00 INFO kernel:
[   74.747364] iwlwifi 0000:01:00.0: loaded firmware version 29.610311.0 op_mode iwlmvm

[w/a patch] It takes about 2 seconds.
..
2018-04-20T01:18:16.454087-07:00 WARNING kernel:
[   75.966860] Timeout waiting for hardware access (CSR_GP_CNTRL 0xffffffff)
..
2018-04-20T01:18:18.602717-07:00 INFO kernel:
[   78.116132] iwlwifi 0000:01:00.0: loaded firmware version 29.610311.0 op_mode iwlmvm
..

Fixes: 49564a80 ("iwlwifi: pcie: remove non-responsive device")
Signed-off-by: Matt Chen <matt.chen@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>