Ensure that ixl_flush() uses a defined register on VFs

In some code that is shared between the ixl(4) and ixlv(4) drivers,
a macro hard-coded a register offset that was not valid on ixlv devices.
Fix this by having each driver define a variable that contains the correct
offset.

Reviewed by:	Eric Joyner <ricera10 AT gmail.com>
MFC after:	3 days
Sponsored by:	Sandvine Inc