BLE socket transport - don't drop large frames.

For large incoming ACL packets, the "current offset" field was rolling
over, making it look like the packet was being dropped.  The fix is to
change the offset member from uint8_t to uint16_t.

X-Original-Commit: 24da43b88ec78642562c5020386d06a26d497955

nimble/transport/socket/src/ble_hci_socket.c

@@ -167,7 +167,7 @@ static struct ble_hci_sock_state {
     struct os_event ev;
     struct os_callout timer;
 
-    uint8_t rx_off;
+    uint16_t rx_off;
     uint8_t rx_data[512];
 } ble_hci_sock_state;