i965/gen6: Fix assign instead of compare in assert

i965/gen6: Fix assign instead of compare in assert

This is from a Coverity defect report.

src/mesa/drivers/dri/i965/brw_vec4_emit.cpp
   268  static void
   269  check_gen6_math_src_arg(struct brw_reg src)
   270  {
   271     /* Source swizzles are ignored. */
   272     assert(!src.abs);
   273     assert(!src.negate);
-> 274     assert(src.dw1.bits.swizzle = BRW_SWIZZLE_XYZW);
   275  }

Reported-by: Vinson Lee <vlee@vmware.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40214
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>