ntb_hw_switchtec: make ntb_mw_set_trans() work when addr == 0

On switchtec_ntb_mw_set_trans() call, when (only) address == 0, it acts as
ntb_mw_clear_trans(). Fix this, since address == 0 and size != 0 is valid
combination for setting translation.

Signed-off-by: Alexander Fomichev <fomichev.ru@gmail.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>