Delay in `read_byte` may be too long

[nomis]

Running on a LOLIN D1 mini I've had to decrease the delay in read_byte to 16 or 17 swim clock cycles or there's always a timeout on bits 6, 7 or 8.

The decimal "indicate which bit failed" offset isn't very usable because stm8flash reports unknown error codes as hex.

--- src/driver/swim.c
+++ src/driver/swim.c
@@ -181,11 +182,11 @@ static int read_byte() {
   uint32_t parity = 0;
   uint32_t i;
   for (i = 0; i < 9; i++) {
-    sync_ccount(next + 18 * SWIM_CLOCK);
+    sync_ccount(next + 17 * SWIM_CLOCK);
     int bit = read_bit(&next);
     if (bit == SWIM_ERROR_READ_BIT_TIMEOUT) {
       // indicate which bit failed (for debugging)
-      return -i - 20;
+      return -i - 0x20;
     }
     result = result << 1 | !!bit;
     parity ^= result;