staging: zcache: fix a compile warning

Fix:

drivers/staging/zcache/zcache-main.c: In function ‘zcache_comp_op’:
drivers/staging/zcache/zcache-main.c:112:2: warning: ‘ret’ may be used uninitial
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/zcache-main.c

@@ -109,6 +109,8 @@ static inline int zcache_comp_op(enum comp_op op,
 	case ZCACHE_COMPOP_DECOMPRESS:
 		ret = crypto_comp_decompress(tfm, src, slen, dst, dlen);
 		break;
+	default:
+		ret = -EINVAL;
 	}
 	put_cpu();
 	return ret;