rpmsg: char: release allocated memory

commit bbe692e349e2a1edf3fe0a29a0e05899c9c94d51 upstream.

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>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>