Fix PWM slice calculation for RP2350 in RP.PWM.To_PWM

The function performed Shift_Right (Pin, 1) but then ignored the shifted
value and overwrote Slice with "Pin and 2#111#". The fix now correctly
uses the already shifted value for the mask operation (Slice and
2#111#).  This matches the RP2350 datasheet and resolves incorrect slice
assignments (e.g. GPIO14 was mapped to slice 6 instead of 7), which
caused missing or flickering PWM outputs when pins shared a slice on the
Pico 2.

Author: krischik

src/devices/rp2350/rp-pwm.adb

@@ -160,9 +160,9 @@ package body RP.PWM is
       Slice    : UInt32 := Shift_Right (Pin, 1);
    begin
       if Pin < 32 then
-         Slice := Pin and 2#111#;
+         Slice := Slice and 2#111#;
       else
-         Slice := 8 + (Pin and 2#11#);
+         Slice := 8 + (Slice and 2#11#);
       end if;
       return PWM_Point'
          (Slice   => PWM_Slice (Slice),