audio_coding/codec/ilbc: Removed usage of WEBRTC_SPL_MUL_16_16_RSFT

The macro is defined as
#define WEBRTC_SPL_MUL_16_16_RSFT(a, b, c) \
    (WEBRTC_SPL_MUL_16_16(a, b) >> (c))

where the latter macro is in C defined as
(For definitions on ARMv7 and MIPS, see common_audio/signal_processing/include/spl_inl_{armv7,mips}.h)

The replacement consists of
- avoiding casts to int16_t if inputs already are int16_t
- adding explicit cast to <type> if result is assigned to <type> (other than int or int32_t)
- minor cleanups like remove of unnecessary parantheses and style changes

BUG=3348, 3353
TESTED=locally on Mac and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/39139004

Cr-Commit-Position: refs/heads/master@{#8544}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8544 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/ilbc/chebyshev.c

@@ -65,7 +65,7 @@ int16_t WebRtcIlbcfix_Chebyshev(
 
   /* tmp1W32 = x*b1 - b2 + f[i]/2 */
   tmp1W32 = WEBRTC_SPL_LSHIFT_W32(b1_high * x, 1) +
-      WEBRTC_SPL_LSHIFT_W32(WEBRTC_SPL_MUL_16_16_RSFT(b1_low, x, 15), 1);
+      WEBRTC_SPL_LSHIFT_W32((b1_low * x) >> 15, 1);
 
   tmp1W32 -= b2;
   tmp1W32 += WEBRTC_SPL_LSHIFT_W32((int32_t)f[i], 13);