i40e: don't overload fields

Overloading the msg_size field in the arq_event_info struct is just a
bad idea. It leads to repeated bugs when the structure is used in a
loop, since the input value (buffer size) is overwritten by the output
value (actual message length).

Fix this by splitting the field into two and renaming to indicate the
actual function of each field.

Since the arq_event struct has now changed, we need to change the drivers
to support this. Note that we no longer need to initialize the buffer size
each time we go through a loop as this value is no longer destroyed by
arq processing.

In the process, we also fix a bug in i40evf_verify_api_ver where the
buffer size was not correctly reinitialized each time through the loop.

Change-ID: Ic7f9633cdd6f871f93e698dfb095e29c696f5581
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Acked-by: Ashish Shah <ashish.n.shah@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>