bluetooth: ots: fix stack overflow

Default size of BT_RX_STACK_SIZE 1024 is not enough for this sample.
Increase it to prevent stack overflow.

Signed-off-by: Pirun Lee <pirun.lee@nordicsemi.no>
diff --git a/samples/bluetooth/peripheral_ots/prj.conf b/samples/bluetooth/peripheral_ots/prj.conf
index 4269e39..dc88dcb 100644
--- a/samples/bluetooth/peripheral_ots/prj.conf
+++ b/samples/bluetooth/peripheral_ots/prj.conf
@@ -11,3 +11,5 @@
 
 CONFIG_LOG=y
 CONFIG_ASSERT=y
+# This sample needs more memory on BT_RX_THREAD
+CONFIG_BT_RX_STACK_SIZE=1536