Ford: fix LateralMotionControl2 checksum (#28147)

* Ford: fix LateralMotionControl2 checksum

* loop is nice

* these are redundant

* need to shift left, before didn't match

* same, but more conventional

* whoops

---------

Co-authored-by: Shane Smiskol <shane@smiskol.com>
old-commit-hash: c59d7d1161

selfdrive/car/ford/fordcan.py

@@ -5,11 +5,15 @@ HUDControl = car.CarControl.HUDControl
 
 
 def calculate_lat_ctl2_checksum(mode: int, counter: int, dat: bytearray):
+  curvature = (dat[2] << 3) | ((dat[3]) >> 5)
+  curvature_rate = (dat[6] << 3) | ((dat[7]) >> 5)
+  path_angle = ((dat[3] & 0x1F) << 6) | ((dat[4]) >> 2)
+  path_offset = ((dat[4] & 0x3) << 8) | dat[5]
+
   checksum = mode + counter
-  checksum += dat[2] + ((dat[3] & 0xE0) >> 5)           # curvature
-  checksum += dat[6] + ((dat[7] & 0xE0) >> 5)           # curvature rate
-  checksum += (dat[3] & 0x1F) + ((dat[4] & 0xFC) >> 2)  # path angle
-  checksum += (dat[4] & 0x3) + dat[5]                   # path offset
+  for sig_val in (curvature, curvature_rate, path_angle, path_offset):
+    checksum += sig_val + (sig_val >> 8)
+
   return 0xFF - (checksum & 0xFF)