celp_math: rename ff_log2 to ff_log2_q15

This name is more descriptive as the function returns a fixed-point
value with 15 fraction bits.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/celp_math.c

@@ -67,7 +67,7 @@ static const uint16_t tab_log2[33] =
   26457,  27294,  28116,  28924,  29719,  30500,  31269,  32025,  32769,
 };
 
-int ff_log2(uint32_t value)
+int ff_log2_q15(uint32_t value)
 {
     uint8_t  power_int;
     uint8_t  frac_x0;