ntb_hw_switchtec: Fix ntb_mw_clear_trans error if size == 0

ntb_mw_set_trans() should work as ntb_mw_clear_trans() when size == 0 and/or
addr == 0. But error in xlate_pos checking condition prevents this.
Fix the condition to make ntb_mw_clear_trans() working.

Fixes: 87d11e64 (NTB: switchtec_ntb: Add memory window support)
Signed-off-by: Alexander Fomichev <fomichev.ru@gmail.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>