misc: fastrpc: Fix an incomplete memory release in fastrpc_rpmsg_probe()

fastrpc_channel_ctx is not freed if misc_register() fails, this would
lead to a memory leak. Fix this leak by adding kfree in misc_register()
error path.

Fixes: 278d56f9

 ("misc: fastrpc: Reference count channel context")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200511162722.2552-1-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>