ibmvnic: Harden device login requests

The VNIC driver's "login" command sequence is the final step
in the driver's initialization process with device firmware,
confirming the available device queue resources to be utilized
by the driver. Under high system load, firmware may not respond
to the request in a timely manner or may abort the request. In
such cases, the driver should reattempt the login command
sequence. In case of a device error, the number of retries
is bounded.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>