rpmsg: fix a comment typo for rpmsg_device_match()

Should be 'a' rather than 'an'.

Signed-off-by: WANG Wenhu <wenhu.wang@vivo.com>
Link: https://lore.kernel.org/r/20200313165049.62907-1-wenhu.wang@vivo.com


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