nfp: make vNIC ctrl memory mapping function reusable

We will soon need to map control vNIC PCI memory as well as data vNIC
memory.  Make the function for mapping areas pointed to by an RTsym
reusable.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>