gfapi: In glfs_set_volfile_server() remove the port number check.

The documentation for glfs_set_volfile_server() api says that the
port zero is a valid value. Specifying 0 uses the default port
number GF_DEFAULT_BASE_PORT. Hence removing the check for port being
zero.

Change-Id: I942a7fe54a418231a438ab67756537df8f28d2de
BUG: 1084964
Signed-off-by: Poornima G <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/7409
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>