Revert "i2c: duplicate TCA9548 control byte"

This reverts commit f265976df6.

artiq/firmware/libboard_misoc/i2c.rs

@@ -183,12 +183,9 @@ mod imp {
         if !write(busno, address << 1)? {
             return Err("PCA9548 failed to ack write address")
         }
-        // Duplicate control byte: one for SCL, one for SDA
-        for _ in 0..2 {
         if !write(busno, channels)? {
             return Err("PCA9548 failed to ack control word")
         }
-        }
         stop(busno)?;
         Ok(())
     }