fir.py: Fix bug identified in iss #2.

Author: peterhinch

README.md

@@ -109,7 +109,7 @@ it runs about three times faster (on the order of 15μs).
 The `fir_py` module uses a closure to enable the function to retain state
 between calls. Usage is as follows:
 ```python
-from fir_py import fir_c
+from fir_py import create_fir
 from array import array
 # 21 tap LPF. Figures from TFilter.
 coeffs = array('i', (-1318, -3829, -4009, -717, 3359, 2177, -3706, -5613,

fir.py

@@ -75,7 +75,7 @@ def fir(r0, r1, r2):
  asr(r2, r7) # Scale result before summing
  add(r0, r2, r0)
  sub(r6, 1)
- bpl(FILT)
+ bne(FILT) # > 0. bpl branched when >= 0
  pop({r3})
  mov(r8, r3) # Restore R8