bnxt: change receive ring space parameters

Modify ring header data split and jumbo parameters to account
for the fact that the design for XDP multibuffer puts close to
the first 4k of data in a page and the remaining portions of
the packet go in the aggregation ring.

v3: Simplified code around initial buffer size calculation

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>