Bluetooth: bt3c_cs: Fix obsolete function

simple_strtol and simple_strtoul are obsolete, both place
use kstrtouint instead.

V2: fix error tmp += tn
V3: fix compile error

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>