nfp: nsp: use fractional size of the buffer

NSP expresses the buffer size in MB and 4 kB blocks.  For small
buffers the kB part may make a difference, so count it in.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>