net/vmxnet3: generate link-state change notifications

net/vmxnet3: generate link-state change notifications

Generate link-state change notifications by listening to interrupts
generated by the device. Make use of the existing
vmxnet3_process_events function that was compiled out, but change it
to call vmxnet3_dev_link_update on a VMXNET3_ECR_LINK event and to not
be so noisy in its log messages.

Enable interrupts on starting the device, using a new helper function,
vmxnet3_enable_intr, based on vmxnet3_disable_intr and validated
against the FreeBSD driver.

Keep track of the number of interrupts registered for to avoid
hardcoding these in vmxnet3_enable/disable_intr and to provision for
any future rxq intr support.

Factor out the guts of vmxnet3_dev_link_update minus the started check
to allow the new function to be called from vmxnet3_dev_start in the
lsc-enabled case to ensure that the link state is correctly set from
the actual state at that point.

Signed-off-by: Robert Shearman <rshearma@brocade.com>