stm32xx-sys: fix typo in EXTI pin configuration

In the Giant Scary Nested Match we had to write to be able to address
the EXTI register array as an array through the PAC, I typo'd the number
base being used to select the field within the register, by reusing 2
from the line above. This is the _base 2 logarithm_ of the number we
actually wanted, 4, because the operations are shift and mod,
respectively.

Wheeee

Anyway, this fixes the situation where EXTI configuration of any pin p
such that p % 4 = 2 or 3 on any port other than A mysteriously senses
a different pin in port A instead.

drv/stm32xx-sys/src/main.rs

@@ -398,7 +398,7 @@ fn main() -> ! {
                 // Process entries that are filled in...
                 if let &Some(ExtiDispatch { port, .. }) = entry {
                     let register = i >> 2;
-                    let slot = i % 2;
+                    let slot = i & 0b11;
 
                     // This is an array of 4-bit fields spread across 4 32-bit
                     // registers. We're indexing them with i. There is really no