nfp: fix incorrect use of mbox in IPsec code

The mailbox configuration mechanism requires writing several registers,
which shouldn't be interrupted, so need lock to avoid race condition.

The base offset of mailbox configuration registers is not fixed, it
depends on TLV caps read from application firmware.

Fixes: 859a497f

 ("nfp: implement xfrm callbacks and expose ipsec offload feature to upper layer")
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>