machine: [rp2] discount scheduling delays in I2C timeouts

The `gosched` call introduce arbitrary long delays in general, and in
TinyGo particular because the goroutine scheduler is cooperative and
doesn't preempt busy (e.g. compute-heavy) goroutines.
This change discounts such delays from the rp2xxx I2C timeout logic.

I tested this change by simulating a busy goroutine:

	go func() {
		for {
      // Busy.
			before := time.Now()
			for time.Since(before) < 100*time.Millisecond {
			}
      // Sleep.
			time.Sleep(100 * time.Millisecond)
		}
	}()

and testing that I2C transfers would no longer time out.

Author: eliasnaur

src/machine/machine_rp2_i2c.go

@@ -338,7 +338,9 @@ func (i2c *I2C) tx(addr uint8, tx, rx []byte, timeout_us uint64) (err error) {
 				return errI2CWriteTimeout // If there was a timeout, don't attempt to do anything else.
 			}
 
+			now := ticks()
 			gosched()
+			deadline += ticks() - now
 		}
 
 		abortReason = i2c.getAbortReason()
@@ -361,7 +363,9 @@ func (i2c *I2C) tx(addr uint8, tx, rx []byte, timeout_us uint64) (err error) {
 					return errI2CWriteTimeout
 				}
 
+				now := ticks()
 				gosched()
+				deadline += ticks() - now
 			}
 			i2c.Bus.IC_CLR_STOP_DET.Get()
 		}