misc: fastrpc: fix memory leak from miscdev->name

misc: fastrpc: fix memory leak from miscdev->name

Fix a memory leak in miscdev->name by using devm_variant

Orignally reported by kmemleak:
    [<ffffff80088b74d8>] kmemleak_alloc+0x50/0x84
    [<ffffff80081e015c>] __kmalloc_track_caller+0xe8/0x168
    [<ffffff8008371ab0>] kvasprintf+0x78/0x100
    [<ffffff8008371c6c>] kasprintf+0x50/0x74
    [<ffffff8008507f2c>] fastrpc_rpmsg_probe+0xd8/0x20c
    [<ffffff80086b63b4>] rpmsg_dev_probe+0xa8/0x148
    [<ffffff80084de50c>] really_probe+0x208/0x248
    [<ffffff80084de2dc>] driver_probe_device+0x98/0xc0
    [<ffffff80084dec6c>] __device_attach_driver+0x9c/0xac
    [<ffffff80084dca8c>] bus_for_each_drv+0x60/0x8c
    [<ffffff80084de64c>] __device_attach+0x8c/0x100
    [<ffffff80084de6e0>] device_initial_probe+0x20/0x28
    [<ffffff80084dcbd0>] bus_probe_device+0x34/0x7c
    [<ffffff80084da32c>] device_add+0x420/0x498
    [<ffffff80084da680>] device_register+0x24/0x2c

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20191009144123.24583-3-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>