target: Fix incorrect inversion of TPGS_EXPLICT_ALUA check

Fix a bug introduced with patch "target: pass sense_reason as a return value"
in for-3.8 code where only target port groups with TPGS_EXPLICT_ALUA set
need to be allowed to perform explictly ALUA.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>