gateware: fix occasional IQ swap in half-band decimator

Prevent resetting internal pointers that manage IQ time-multiplexing
within the FIR filter.

Author: mndza

firmware/fpga/build/praline_fpga.bin

@@ -132,8 +132,6 @@ def elaborate(self, platform):
 
                 # Mode switch logic.
                 with m.If(~self.enable):
-                    m.d.sync += input_idx.eq(0)
-                    m.d.sync += output_idx.eq(0)
                     m.d.sync += odd.eq(0)
                     m.d.sync += even_valid.eq(0)
                     m.next = "BYPASS"