raw/ntb: clear all valid doorbell bits on init

raw/ntb: clear all valid doorbell bits on init

Before registering the doorbell interrupt handler callback function,
all the valid doorbell bits within the NTB private data struct should
be cleared to avoid the confusion of the handshake timing sequence
diagram when setting up the NTB connection in back-to-back mode.

Fixes: 62012a76811e ("raw/ntb: add handshake process")
Cc: stable@dpdk.org
Signed-off-by: Junfeng Guo <junfeng.guo@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>