rpmsg: remove pointless OOM prints

These types of error prints are superfluous. The system will
pick up on OOM issues and let the user know. While at this,
fix the usage of using a structure instead of the actual
variable in one of the allocations.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>