rpmsg: char: release allocated memory

In rpmsg_eptdev_write_iter, if copy_from_iter_full fails the allocated
buffer needs to be released.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
