bnxt_en: Parameterize RX buffer offsets.

Convert the global constants BNXT_RX_OFFSET and BNXT_RX_DMA_OFFSET to
device parameters.  This will make it easier to support XDP with
headroom support which requires different RX buffer offsets.

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