rpmsg: glink: smem: Support rx peak for size less than 4 bytes

The current rx peak function fails to read the data if size is
less than 4bytes.

Use memcpy_fromio to support data reads of size less than 4 bytes.

Cc: stable@vger.kernel.org
Fixes: f0beb4ba ("rpmsg: glink: Remove chunk size word align warning")
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>