rpmsg: smd: allow opening rpm_requests even if already opened

On msm8953 the channel seems to be already opened when booting Linux but
we still need to open it for communication with regulators etc.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220220201909.445468-6-luca@z3ntu.xyz