soc: qcom: glink: Make error codes negative in glink_smem_native_xprt

Positive error codes are returned in some places in
glink_smem_native_xprt.c. Per convention, these should be negative.

Return negative error codes instead.

Change-Id: Ib09f7216474554517aa477c517ce4b83dc87f7cc
Signed-off-by: Steven Cahail <scahail@codeaurora.org>