rpmsg: char: treat rpmsg_trysend() ENOMEM as EAGAIN

rpmsg_trysend() returns -ENOMEM when no rpmsg buffer can be allocated.
this causes write to fail with this error as opposed to -EAGAIN.
this is what user space applications (and libraries like boost.asio)
would expect when using normal character devices.

Signed-off-by: Tim Blechmann <tim@klingt.org>
Cc: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220313024541.1579848-2-tim@klingt.org