rpmsg: char: Migrate to iter versions of read and write

In order to be able to use the aio interface for writing to a rpmsg_char
the write_iter function must be implemented, so migrate to iter version
for read and write functions.

Regular read and write uses the iter methods if present and is as such
unaffected.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>