Clear i2c abort reason less often. (raspberrypi#2026)

It seems to be possible to get stuck in the loop which is checking for
abort. It can take 100s of iterations before an abort happens and on
each iteration we're clearing the abort interrupt even when it's not
required. If we only clear the abort when needed the lockup doesn't seem
to be reproducible.

Fixes raspberrypi#2025

Author: peterharperuk

src/rp2_common/hardware_i2c/i2c.c

@@ -298,7 +298,10 @@ static int i2c_read_blocking_internal(i2c_inst_t *i2c, uint8_t addr, uint8_t *ds
 
         do {
             abort_reason = i2c->hw->tx_abrt_source;
-            abort = (bool) i2c->hw->clr_tx_abrt;
+            if (i2c->hw->raw_intr_stat & I2C_IC_RAW_INTR_STAT_TX_ABRT_BITS) {
+                abort = true;
+                i2c->hw->clr_tx_abrt;
+            }
             if (timeout_check) {
                 timeout = timeout_check(ts, false);
                 abort |= timeout;