extmod/machine_i2c: Use writes not reads in i2c.scan().

As per discussion in #2449, using write requests instead of read requests
for I2C.scan() seems to support a larger number of devices, especially
ones that are write-only.  Even a read-only I2C device has to implement
writes in order to be able to receive the address of the register to read.

extmod/machine_i2c.c

@@ -272,7 +272,7 @@ STATIC mp_obj_t machine_i2c_scan(mp_obj_t self_in) {
     // 7-bit addresses 0b0000xxx and 0b1111xxx are reserved
     for (int addr = 0x08; addr < 0x78; ++addr) {
         mp_hal_i2c_start(self);
-        int ack = mp_hal_i2c_write_byte(self, (addr << 1) | 1);
+        int ack = mp_hal_i2c_write_byte(self, (addr << 1));
         if (ack) {
             mp_obj_list_append(list, MP_OBJ_NEW_SMALL_INT(addr));
         }