Faster isr

Before:
```
Sketch uses 3,606 bytes (11%) of program storage space. Maximum is 32,256 bytes.

   text    data     bss     dec     hex filename
   3496     110    1169    4775    12a7 /tmp/build3790807526441479191.tmp/spi_speed_test.cpp.elf
```
After:

```
Sketch uses 3,606 bytes (11%) of program storage space. Maximum is 32,256 bytes.

   text    data     bss     dec     hex filename
   3496     110    1169    4775    12a7 /tmp/build3790807526441479191.tmp/spi_speed_test.cpp.elf
```
The checking if the target ISR is set is removed
and traded in for setting the initial array values and an empty function.
This gives you a speed gain when an ISR comes in, without increasing the size of the binary.