ANDROID: fix up rpmsg_device ABI break

In commit e70898ae ("rpmsg: Fix kfree() of static memory on setting
driver_override") a pointer was changed to const, which messes with the
CRC and ABI checks.  As the code is fine if this is left as not-const,
just put it back to preserve the abi.

Bug: 161946584
Fixes: e70898ae ("rpmsg: Fix kfree() of static memory on setting driver_override")
Change-Id: I9a87b9cf412191d9872b48f1f876a81df6701de0
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>