hash_sse42.c: Fix static analysis error.

The error at this line was:
"Although the value stored to 'crc' is used in the enclosing expression,
the value is never actually read from 'crc'."

BUG=aomedia:2957

Change-Id: Icfc63bb55ade93fa24b611098e2aa0c434f5d1ca

av1/encoder/x86/hash_sse42.c

diff --git a/av1/encoder/x86/hash_sse42.c b/av1/encoder/x86/hash_sse42.c
index 65fa463..0becdf9 100644
--- a/av1/encoder/x86/hash_sse42.c
+++ b/av1/encoder/x86/hash_sse42.c
@@ -47,5 +47,5 @@
   CALC_CRC(_mm_crc32_u32, crc, uint32_t, buf, len);
   CALC_CRC(_mm_crc32_u16, crc, uint16_t, buf, len);
   CALC_CRC(_mm_crc32_u8, crc, uint8_t, buf, len);
-  return (crc ^= 0xFFFFFFFF);
+  return (crc ^ 0xFFFFFFFF);
 }