if_ntb: MFV c92ba3c5: invalid buf pointer in multi-MW setups

Order of operations issue with the QP Num and MW count, which would
result in the receive buffer pointer being invalid if there are more
than 1 MW. Corrected with parenthesis to enforce the proper order of
operations.

Reported by: John I. Kading <John.Kading@gd-ms.com>
Reported by: Conrad Meyer <cem@FreeBSD.org>
Authored by: Jon Mason <jdmason@kudzu.us>
Obtained from: Linux (Dual BSD/GPL driver)
Sponsored by: EMC / Isilon Storage Division