Improve panic message when requesting frequency higher than clock

Previously it would panic with message "unreachable", which isn't
particularly clear about what the problem is and how to fix it.

embassy-stm32/src/spi/mod.rs

@@ -700,7 +700,7 @@ use vals::Mbr as Br;
 
 fn compute_baud_rate(clocks: Hertz, freq: Hertz) -> Br {
     let val = match clocks.0 / freq.0 {
-        0 => unreachable!(),
+        0 => panic!("You are trying to reach a frequency higher than the clock"),
         1..=2 => 0b000,
         3..=5 => 0b001,
         6..=11 => 0b010,