misc: fastrpc: fix potential fastrpc_invoke_ctx leak

fastrpc_invoke_ctx can have refcount of 2 in error path where
rpmsg_send() fails to send invoke message. decrement the refcount
properly in the error path to fix this leak.

This also fixes below static checker warning:

drivers/misc/fastrpc.c:990 fastrpc_internal_invoke()
warn: 'ctx->refcount.refcount.ref.counter' not decremented on lines: 990.

Fixes: c68cfb71

 ("misc: fastrpc: Add support for context")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200512110930.2550-1-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>