soc: qcom: glink: update glink_rpm_rx_poll return doc

glink_rpm_rx_poll() return documentation does not document return values
greater than 0.

Update documentation to match actual behavior.

Change-Id: Ic975423bef881ef4b6e90723df9c3d0caca3697b
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>