nfp: centralize the buffer size calculation

Add a helper function to calculate the buffer size at run time.
Buffer lengths will now depend on the FW prepend configuration
instead of assuming the most space consuming configuration and
defaulting to 2k buffers at initialization time.

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