ibmvnic: enhance resetting status check during module exit

Based on the discussion with Sukadev Bhattiprolu and Dany Madden,
we believe that checking adapter->resetting bit is preferred
since RESETTING state flag is not as strict as resetting bit.
RESETTING state flag is removed since it is verbose now.

Fixes: 7d7195a0

 ("ibmvnic: Do not process device remove during device reset")
Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>