Clean up ext_interp

1) Simplify macro branch
2) Improve the naming of interp_filter read/write functions

Change-Id: I0ae290b08a8b058359874d91da468062706ab2d5
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 91c2b3a..9de00fc 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2643,7 +2643,7 @@
   }
 }
 
-static InterpFilter read_interp_filter(struct aom_read_bit_buffer *rb) {
+static InterpFilter read_frame_interp_filter(struct aom_read_bit_buffer *rb) {
   return aom_rb_read_bit(rb) ? SWITCHABLE
                              : aom_rb_read_literal(rb, LOG_SWITCHABLE_FILTERS);
 }
@@ -3938,7 +3938,7 @@
 #endif
 
       cm->allow_high_precision_mv = aom_rb_read_bit(rb);
-      cm->interp_filter = read_interp_filter(rb);
+      cm->interp_filter = read_frame_interp_filter(rb);
 
       for (i = 0; i < INTER_REFS_PER_FRAME; ++i) {
         RefBuffer *const ref_buf = &cm->frame_refs[i];