bnxt: refactor bnxt_rx_xdp to separate xdp_init_buff/xdp_prepare_buff

Move initialization of xdp_buff outside of bnxt_rx_xdp to prepare
for allowing bnxt_rx_xdp to operate on multibuffer xdp_buffs.

v2: Fix uninitalized variables warning in bnxt_xdp.c.
v3: Add new define BNXT_PAGE_MODE_BUF_SIZE

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>