rpmsg: glink: Fix missing mutex_init() in qcom_glink_alloc_channel()

qcom_glink_alloc_channel() allocates the mutex but not initialize it.
Use mutex_init() on it to initialize it correctly.

This is detected by Coccinelle semantic patch.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>